docs: Add ARM64 platform considerations to instruction files

Add comprehensive ARM64 section to both CLAUDE.md and
copilot-instructions.md covering:

- Architecture detection (_M_ARM64 vs _WIN64 distinction)
- NP3_BUILD_ARM64 helper macro in TypeDefs.h
- ARM64 rendering defaults (DirectWriteRetain, WS_EX_COMPOSITED)
- ARM64 build config requirements (CETCompat, TargetMachine, _WIN64)
- GrepWin x64 emulation on ARM64
- Theme change flickering prevention (WM_SETREDRAW lock)
- ARM 32-bit not supported (solution config maps to Win32)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/copilot-instructions.md

@@ -48,7 +48,7 @@ GitHub Actions workflow at `.github/workflows/build.yml` builds all four platfor
 
 ## Architecture
 
-Notepad3 is a Win32 desktop text editor built on the **Scintilla** editing component with **Lexilla** for syntax highlighting. It ships with the companion tool **MiniPath** (file browser) and integrates with the external **grepWin** tool (file search/grep).
+Notepad3 is a Win32 desktop text editor built on the **Scintilla** editing component with **Lexilla** for syntax highlighting. It ships with the companion tool **MiniPath** (file browser) and integrates with the external **grepWin** tool (file search/grep) via pre-built portable executables.
 
 ### Core modules (in `src\`)
 
@@ -75,6 +75,22 @@ Notepad3 is a Win32 desktop text editor built on the **Scintilla** editing compo
 - **`src\uthash\`** — Hash table library (C macros)
 - **`src\crypto\`** — Rijndael/SHA-256 for AES-256 encryption
 
+### grepWin Integration (`grepWin\`)
+
+grepWin is an **external** file search/grep tool — it is **not** built from source as part of Notepad3. Pre-built portable executables and translation files are stored in the repository:
+
+- **`grepWin\portables\`** — `grepWin-x86_portable.exe`, `grepWin-x64_portable.exe`, `LICENSE.txt`
+- **`grepWin\translations\`** — `*.lang` translation files (e.g. `German.lang`, `French.lang`)
+
+At runtime (`src\Dialogs.c`), Notepad3 searches for grepWin in this order:
+1. `Settings2.GrepWinPath` (user-configured INI setting)
+2. `<ModuleDirectory>\grepWin\grepWin-x64_portable.exe` (or x86) — portable layout
+3. `%APPDATA%\Rizonesoft\Notepad3\grepWin\` — installed layout
+
+Language mapping (`src\MuiLanguage.c`): `grepWinLangResName[]` maps Notepad3 locale names (e.g. `de-DE`) to grepWin `.lang` filenames (e.g. `German.lang`). The language file path is written to `grepwin.ini` before launching.
+
+Portable build scripts (`Build\make_portable_*.cmd`) package grepWin into a `grepWin\` subdirectory in the archive containing both portable executables, the license, and all `*.lang` translations.
+
 ### Syntax highlighting lexers (`src\StyleLexers\`)
 
 Each language has its own `styleLexXXX.c` file (~50+ languages). All lexers follow the `EDITLEXER` struct pattern defined in `EditLexer.h`:
@@ -138,6 +154,33 @@ URL hotspot regex is defined at `src\Edit.c:108` (`HYPLNK_REGEX_FULL` macro). It
 
 Windows 10/11 dark mode via IAT (Import Address Table) hooks. Includes stub DLLs for uxtheme and user32.
 
+### ARM64 Platform Considerations
+
+**Supported platforms**: Win32 (x86), x64, x64_AVX2, ARM64. ARM 32-bit is **not** supported (the `Release|ARM` solution config maps to Win32).
+
+#### Architecture detection
+
+Use `#if defined(_M_ARM64)` or the helper macro `NP3_BUILD_ARM64` (defined in `src\TypeDefs.h`) for ARM64-specific code paths. **Important**: both ARM64 and x64 define `_WIN64`, so use `_M_ARM64` when you need to distinguish ARM64 from x64.
+
+#### ARM64 rendering defaults
+
+ARM64 defaults to `SC_TECHNOLOGY_DIRECTWRITERETAIN` (value 2) instead of `SC_TECHNOLOGY_DIRECTWRITE` (value 1) to preserve the Direct2D back buffer between frames. This avoids flickering on Qualcomm Adreno GPUs and the Win11 25H2 DWM compositor. The main window also uses `WS_EX_COMPOSITED` on ARM64 for system-level double-buffering. Users can override via `RenderingTechnology` in the INI file or the View menu.
+
+#### ARM64 build configuration
+
+- `CETCompat` must be `false` for ARM64 (CET is x86/x64 only)
+- `TargetMachine` must be `MachineARM64` in all ARM64 linker sections
+- `_WIN64` must be defined in preprocessor definitions for all ARM64 configurations
+- Build fix scripts in `Build\scripts\`: `FixARM64CETCompat.ps1`, `FixARM64CrossCompile.ps1`, `FixARM64OutDir.ps1`
+
+#### GrepWin on ARM64
+
+No native ARM64 grepWin build exists. The ARM64 build uses `grepWin-x64_portable.exe` which runs via x64 emulation on Windows ARM64. The binary selection in `src\Notepad3.c` uses `#if defined(_M_ARM64)` to handle this explicitly.
+
+#### Theme change flickering prevention
+
+`MsgThemeChanged()` in `src\Notepad3.c` wraps all heavy operations (bar recreation, lexer reset, restyling) in `WM_SETREDRAW FALSE/TRUE` to suppress intermediate repaints and performs a single `RedrawWindow()` at the end. DarkMode `RedrawWindow()` calls in `ListViewUtil.hpp` omit `RDW_ERASE` to avoid background erase flashes during theme transitions.
+
 ## Conventions
 
 ### Code style

CLAUDE.md

@@ -4,7 +4,7 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 
 ## Project Overview
 
-Notepad3 is a Windows-only Win32 desktop text editor (C/C++) built on **Scintilla** (editing component) and **Lexilla** (syntax highlighting). It ships with the companion tool **MiniPath** (file browser, Ctrl+M) and integrates with the external **grepWin** tool (file search/grep, Ctrl+Shift+F). Licensed under BSD 3-Clause.
+Notepad3 is a Windows-only Win32 desktop text editor (C/C++) built on **Scintilla** (editing component) and **Lexilla** (syntax highlighting). It ships with the companion tool **MiniPath** (file browser, Ctrl+M) and integrates with the external **grepWin** tool (file search/grep, Ctrl+Shift+F) via pre-built portable executables. Licensed under BSD 3-Clause.
 
 ## Build Commands
 
@@ -85,6 +85,22 @@ MainWndProc (Notepad3.c)
 | `src\uthash\` | uthash | Hash table / dynamic array macros |
 | `src\crypto\` | Rijndael/SHA-256 | AES-256 encryption |
 
+### grepWin Integration (`grepWin\`)
+
+grepWin is an **external** file search/grep tool — it is **not** built from source as part of Notepad3. Pre-built portable executables and translation files are stored in the repository:
+
+- **`grepWin\portables\`** — `grepWin-x86_portable.exe`, `grepWin-x64_portable.exe`, `LICENSE.txt`
+- **`grepWin\translations\`** — `*.lang` translation files (e.g. `German.lang`, `French.lang`)
+
+At runtime (`src\Dialogs.c`), Notepad3 searches for grepWin in this order:
+1. `Settings2.GrepWinPath` (user-configured INI setting)
+2. `<ModuleDirectory>\grepWin\grepWin-x64_portable.exe` (or x86) — portable layout
+3. `%APPDATA%\Rizonesoft\Notepad3\grepWin\` — installed layout
+
+Language mapping (`src\MuiLanguage.c`): `grepWinLangResName[]` maps Notepad3 locale names (e.g. `de-DE`) to grepWin `.lang` filenames (e.g. `German.lang`). The language file path is written to `grepwin.ini` before launching.
+
+Portable build scripts (`Build\make_portable_*.cmd`) package grepWin into a `grepWin\` subdirectory in the archive containing both portable executables, the license, and all `*.lang` translations.
+
 ### Syntax Lexers (`src\StyleLexers\`)
 
 50+ languages, each in a `styleLexXXX.c` file. All follow the `EDITLEXER` struct pattern from `EditLexer.h`:
@@ -190,7 +206,32 @@ URL hotspot regex is defined at `src\Edit.c:108` (`HYPLNK_REGEX_FULL` macro). It
 
 Windows 10/11 dark mode via IAT (Import Address Table) hooks. Includes stub DLLs for uxtheme and user32.
 
-## Code Conventions
+### ARM64 Platform Considerations
+
+**Supported platforms**: Win32 (x86), x64, x64_AVX2, ARM64. ARM 32-bit is **not** supported (the `Release|ARM` solution config maps to Win32).
+
+#### Architecture detection
+
+Use `#if defined(_M_ARM64)` or the helper macro `NP3_BUILD_ARM64` (defined in `src\TypeDefs.h`) for ARM64-specific code paths. **Important**: both ARM64 and x64 define `_WIN64`, so use `_M_ARM64` when you need to distinguish ARM64 from x64.
+
+#### ARM64 rendering defaults
+
+ARM64 defaults to `SC_TECHNOLOGY_DIRECTWRITERETAIN` (value 2) instead of `SC_TECHNOLOGY_DIRECTWRITE` (value 1) to preserve the Direct2D back buffer between frames. This avoids flickering on Qualcomm Adreno GPUs and the Win11 25H2 DWM compositor. The main window also uses `WS_EX_COMPOSITED` on ARM64 for system-level double-buffering. Users can override via `RenderingTechnology` in the INI file or the View menu.
+
+#### ARM64 build configuration
+
+- `CETCompat` must be `false` for ARM64 (CET is x86/x64 only)
+- `TargetMachine` must be `MachineARM64` in all ARM64 linker sections
+- `_WIN64` must be defined in preprocessor definitions for all ARM64 configurations
+- Build fix scripts in `Build\scripts\`: `FixARM64CETCompat.ps1`, `FixARM64CrossCompile.ps1`, `FixARM64OutDir.ps1`
+
+#### GrepWin on ARM64
+
+No native ARM64 grepWin build exists. The ARM64 build uses `grepWin-x64_portable.exe` which runs via x64 emulation on Windows ARM64. The binary selection in `src\Notepad3.c` uses `#if defined(_M_ARM64)` to handle this explicitly.
+
+#### Theme change flickering prevention
+
+`MsgThemeChanged()` in `src\Notepad3.c` wraps all heavy operations (bar recreation, lexer reset, restyling) in `WM_SETREDRAW FALSE/TRUE` to suppress intermediate repaints and performs a single `RedrawWindow()` at the end. DarkMode `RedrawWindow()` calls in `ListViewUtil.hpp` omit `RDW_ERASE` to avoid background erase flashes during theme transitions.
 
 ### Formatting