Add LZ77 compression to PNG encoder, fix screenshot hook timing

PNG encoder: replace literals-only fixed Huffman with stb-style LZ77
matching. Hash-table chain depth scales with compression level (1-9),
giving a smooth tradeoff from fast/light to slow/best. Lazy matching
cancels a match if the next position finds a longer one. Falls back
to store blocks if compressed output is larger than raw.

Screenshot: move hook install from loadScriptFunctionsDetour to
engineInitDetour (GameEngine_MainInitialize at 0x46a400) so it fires
after all DLL_PROCESS_ATTACH hooks. Restore original CTgaFile::Write
prologue before hooking to avoid chaining through UnitXP. Capture
compression level at enqueue time so each queued shot reflects the
quality setting at the moment it was taken. Counter uses hex suffix
(0-F, max 16/sec) and resets each second.

src/main.zig

@@ -398,6 +398,28 @@ fn callLoadFileListWithIncludes(toc_path: [*:0]const u8, md5ctx: *[88]u8, error_
     );
 }
 
+// =============================================================================
+// Hook: GameEngine_MainInitialize (0x46a400)
+// __stdcall(void) — prologue: 55 8B EC 83 EC 28 = 6 bytes, no fixups
+//
+// Called once from InitializeAllSubsystems during the init callback inside
+// WinMain. Fires after Lua env, UI frames, event tables, and DB tables are
+// set up, but before the event loop starts frame callbacks (i.e. before the
+// login screen renders and screenshots become possible).
+//
+// We install the screenshot hook here to guarantee it runs AFTER all
+// DLL_PROCESS_ATTACH hooks (including UnitXP's CTgaFile::Write hook),
+// making us the outermost detour in the hook chain.
+// =============================================================================
+
+var engine_init_hook: hook.Hook = .{};
+
+fn engineInitDetour() callconv(sc) void {
+    const orig = engine_init_hook.getTrampoline(*const fn () callconv(sc) void);
+    orig();
+    screenshot.installHook();
+}
+
 // =============================================================================
 // Hook: CGGameUI_Shutdown (0x490BD0)
 // Prologue: 56 E8 7A 83 FC FF = 6 bytes, fixup at offset 1
@@ -433,8 +455,9 @@ fn install() void {
         load_addons_hook.activate(@intFromPtr(thunk));
     }
 
-    // 5. Screenshot hook — async PNG capture replacing TGA write
-    screenshot.installHook();
+    // 5. GameEngine_MainInitialize — install screenshot hook after all DLLs load
+    //    Fires once inside WinMain, before the login screen renders.
+    _ = engine_init_hook.install(0x46a400, 6, @intFromPtr(&engineInitDetour), &.{});
 
     // 6. CGGameUI_Shutdown — cleanup
     _ = shutdown_hook.install(0x490BD0, 6, @intFromPtr(&shutdownDetour), &.{1});
@@ -442,6 +465,7 @@ fn install() void {
 
 fn uninstall() void {
     shutdown_hook.remove();
+    engine_init_hook.remove();
     screenshot.removeHook();
     load_addons_hook.remove();
     lsf_hook.remove();