stdlib: relocate modules under library/

- examples/modules/ -> library/modules/ (top-level, no more
  symlink hacks in consumer projects)
- compiler discovers stdlib via _NSGetExecutablePath / readlink
  /proc/self/exe; searches dev layout (../../library), install
  layout (../library), and alongside-binary fallback
- SX_STDLIB_PATH env var overrides for tests / dev convenience
- SX_DEBUG_STDLIB env var dumps the discovery results
- build.zig installs library/ alongside the binary
- Compilation gains stdlib_paths field threaded through resolveImports
- 50 tests pass; consumer projects can now build from any cwd

library/modules/testpkg/math.sx

@@ -0,0 +1,2 @@
+add :: (a: s32, b: s32) -> s32 { a + b; }
+mul :: (a: s32, b: s32) -> s32 { a * b; }