platform/sdl3: chdir to .app bundle on macOS so CWD-relative assets resolve

A macOS .app launched with CWD=/ (Finder/open) could not find CWD-relative
assets (read_file_bytes("assets/...")) and crashed in stbtt with a null font.
SdlPlatform.init now chdirs to SDL_GetBasePath() when running from inside a
.app bundle (detected by ".app" in the base path), mirroring uikit.sx s iOS
chdir_to_bundle. Gated so the sx run dev flow (binary not bundled) keeps the
project CWD. Verified: direct-exec with CWD=/ now stays alive (was: instant
stbtt segfault). Filed issue 0051 with the analysis.

Note: launching via Finder/open additionally triggers Gatekeeper App
Translocation for the dev-signed bundle (separate code-signing concern, not
the asset path).

issues/0051-macos-bundle-assets-cwd-relative.md

@@ -0,0 +1,110 @@
+# Symptom
+
+A bundled macOS `.app` built with `sx build` crashes on launch when started
+via Finder double-click or `open Foo.app`, but runs fine when launched from a
+shell whose CWD is the bundle directory.
+
+Observed: `open sx-out/macos/SxChess.app` → process exits within ~1s (segfaults
+inside `stbtt_ScaleForPixelHeight` because the font buffer is null — the asset
+wasn't found).
+Expected: double-click / `open` launches the app and it finds its bundled
+assets, same as on iOS.
+
+Root: assets are loaded with **CWD-relative** paths (e.g.
+`"assets/fonts/default.ttf"`), but Finder/`open` start a GUI app with `CWD=/`,
+so the relative path resolves against `/` and the file is missing.
+
+# Reproduction
+
+Any consumer that bundles an `assets/` dir and loads from it by relative path.
+Minimal shape (real case: `/Users/agra/projects/game`):
+
+```sx
+// main.sx — loads assets by CWD-relative path
+g_pipeline.init_font("assets/fonts/default.ttf", 32.0, dpi);   // -> read_file_bytes
+g_chess_game.pieces.load("assets/chess/pieces.png", gpu);      // -> read_file_bytes
+```
+
+```sh
+cd game && sx build main.sx          # produces sx-out/macos/SxChess.app (assets copied in)
+
+# Works (CWD = bundle dir, so "assets/..." resolves):
+cd sx-out/macos/SxChess.app && ./SxChess
+
+# Fails (CWD = /, asset not found -> null buffer -> stbtt segfault):
+open sx-out/macos/SxChess.app
+# or double-click in Finder
+```
+
+`add_asset_dir("assets", "assets")` in `build.sx` correctly copies the tree
+into the flat `.app` (binary at `SxChess.app/SxChess`, assets at
+`SxChess.app/assets/...`), so the files ARE present in the bundle — they're just
+not found because the lookup is CWD-relative and CWD isn't the bundle.
+
+# Root cause
+
+The macOS SDL startup never reorients CWD (or the asset root) to the bundle.
+`SdlPlatform.init` ([library/modules/platform/sdl3.sx:35](../library/modules/platform/sdl3.sx#L35))
+calls `SDL_Init(SDL_INIT_VIDEO)` and creates the window but does no `chdir`,
+so `read_file_bytes`
+([library/modules/ui/glyph_cache.sx:202](../library/modules/ui/glyph_cache.sx#L202),
+and the chess `#foreign read_file_bytes`) opens paths relative to whatever CWD
+the launcher set — `/` under Finder/`open`.
+
+The other platforms already handle this:
+- **iOS**: [library/modules/platform/uikit.sx:346](../library/modules/platform/uikit.sx#L346)
+  explicitly `chdir`s to the bundle's `resourcePath()` at startup, with the
+  comment "iOS apps start with CWD=/. chdir to the bundle's resourcePath so the
+  [assets resolve]".
+- **Android**: [library/modules/platform/android.sx:71](../library/modules/platform/android.sx#L71)
+  routes `read_file_bytes` through `AAssetManager` so paths resolve against the
+  APK assets regardless of CWD.
+
+macOS has neither — so it only works by accident when launched from a shell
+sitting in the bundle dir.
+
+# Investigation prompt
+
+For a fresh session picking this up:
+
+The fix mirrors the iOS precedent. In `SdlPlatform.init`
+([sdl3.sx:35](../library/modules/platform/sdl3.sx#L35)), before any asset is
+loaded, reorient to the bundle's resource directory **on macOS only** (leave
+wasm/emscripten and the dev `sx run` path alone — those legitimately want the
+project CWD).
+
+Recommended approach — `SDL_GetBasePath()`:
+- SDL3 `SDL_GetBasePath()` returns the directory containing the executable
+  (for a `.app`, that's `SxChess.app/` where the assets were copied). `chdir`
+  to it at the top of `init` when `BuildOptions.is_macos` (gate so `sx run`
+  during development isn't affected — or gate on "the base path differs from
+  CWD and contains an `assets/` dir").
+- Add the `#foreign` decl for `SDL_GetBasePath` (returns `*u8`, SDL-owned) and
+  call `chdir` (already used by uikit.sx — reuse the same `#foreign`).
+
+Alternative (no SDL dependency): `_NSGetExecutablePath` + `dirname`, same as a
+plain macOS resolve. SDL_GetBasePath is simpler and already links SDL3.
+
+Things to verify / watch:
+- Don't chdir for the `sx run <file>` (JIT) dev flow or for wasm — only the
+  bundled AOT macOS app. The cleanest gate is the bundle context; if `init`
+  can't see `BuildOptions`, gate on `SDL_GetBasePath()` returning a path that
+  ends in `.app/` (bundled) vs the build dir (dev).
+- After chdir, the existing `"assets/..."` relative loads resolve unchanged —
+  no call-site changes needed in consumers (chess, glyph_cache).
+- Confirm the iOS path (uikit, doesn't use SdlPlatform) is untouched.
+
+# Verification
+
+```sh
+cd game && sx build main.sx
+open sx-out/macos/SxChess.app            # must launch and stay up (board + pieces render)
+# screenshot / pgrep -lf sx-out/macos/SxChess  -> alive after a few seconds
+```
+
+Before the fix: `open` → exits ~1s (stbtt segfault, null font buffer).
+After: `open` and Finder double-click both launch and render, matching the
+`cd bundle && ./SxChess` behavior.
+
+Also re-run host + cross suites to confirm no platform-module regression:
+`zig build && zig build test && bash tests/run_examples.sh`.