Modularize prose_code.c into 12 modules + shared header

Split the monolithic 6,327-line prose_code.c into logically organized
modules following MODULARIZATION.md:

- prose_code.h: shared header with all types, macros, externs, prototypes
- buffer.c: arena allocator, gap buffer, line cache, wrap cache, undo stack
- theme.c: dark/light themes and DPI scaling
- spell.c: Windows ISpellChecker COM integration with static GUIDs
- syntax.c: C and Markdown tokenizers, keyword hash table
- document.c: document lifecycle, line/column math, gutter width
- editor.c: text editing ops, clipboard, undo/redo, cursor movement
- search.c: find/replace with match highlighting
- menu.c: menu definitions and action dispatch
- file_io.c: file I/O, atomic saves, autosave/crash recovery, tab management
- render.c: all GDI rendering (titlebar, tabs, editor, minimap, stats)
- wndproc.c: window procedure, input handling, bracket matching
- main.c: entry point, app icon, EditorState global

Also adds Makefile and GitHub Actions CI workflow for cross-compilation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: asystemoffields

.github/workflows/build.yml

@@ -0,0 +1,24 @@
+name: Build
+on:
+  push:
+    branches: [ main, master, 'claude/**' ]
+  pull_request:
+    branches: [ main, master ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install MinGW-w64
+        run: sudo apt-get update && sudo apt-get install -y gcc-mingw-w64-x86-64
+
+      - name: Build
+        run: make CC=x86_64-w64-mingw32-gcc
+
+      - name: Upload artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: prose_code
+          path: prose_code.exe

Makefile

@@ -0,0 +1,25 @@
+CC      ?= x86_64-w64-mingw32-gcc
+WINDRES ?= x86_64-w64-mingw32-windres
+CFLAGS  = -O2 -Wall -Wextra -Wno-unused-parameter -municode \
+          -DUNICODE -D_UNICODE -DCOBJMACROS
+LDFLAGS = -mwindows -municode
+LIBS    = -lgdi32 -lcomdlg32 -lcomctl32 -lshell32 -lole32 \
+          -lshlwapi -ldwmapi -luxtheme
+
+SRCS    = main.c buffer.c theme.c spell.c syntax.c document.c \
+          editor.c search.c menu.c file_io.c render.c wndproc.c
+OBJS    = $(SRCS:.c=.o)
+TARGET  = prose_code.exe
+
+all: $(TARGET)
+
+$(TARGET): $(OBJS)
+	$(CC) $(LDFLAGS) -o $@ $^ $(LIBS)
+
+%.o: %.c prose_code.h
+	$(CC) $(CFLAGS) -c -o $@ $<
+
+clean:
+	rm -f $(OBJS) $(TARGET)
+
+.PHONY: all clean