feat(stdlib): add Fs.* / Path.* / Mime.* — filesystem, paths, content types (#10)

A cohesive filesystem & IO library — the foundation for saves, config, mods, and
asset loading — wrapping the bare file_* builtins into one safe, ergonomic API a
non-expert can use without touching a file descriptor or a byte buffer.

  Path.*  join / dir / base / ext / stem / normalize   (pure lexical string ops)
  Fs.*    exists / is_dir / read_text / write_text / append_text / remove /
          size / mkdir / copy / list
  Mime.*  of (extension table) / sniff (magic bytes: PNG/JPEG/GIF/PDF)

Pure string IR for Path.*; libc (fopen/access/mkdir/rename/opendir…) for Fs.*;
C-free, emitted on demand (g_uses_fsrt). Safety and determinism baked in:
- write_text and copy are atomic (write a temp file, then rename over the target)
  so a crash mid-write never corrupts the previous file;
- mkdir creates parents (mkdir -p);
- list is sorted for a stable, reproducible directory walk;
- fallible calls return values (null / false / -1), never crashes — ready for a
  first-class try/else when the error-handling work lands.

Complements Os.* (#21): Os supplies per-user locations, Fs the operations. v1
targets the native macOS/BSD filesystem with "/" separators; Windows separators,
a sandboxed wasm virtual FS, recursive directory copy, and richer magic-byte
sniffing are documented follow-ups.

- examples/library/fs.ludic: 32 assertions across pure Path ops (incl. normalize
  resolving ./ .. and duplicate slashes), a real create/read/append/copy/list/
  remove cycle under build/, and Mime by-extension + by-magic (GIF signature vs a
  .bin extension). Wired into `x test` (now 56 passed).
- docs: new Path, Fs, and Mime sections + 18 per-symbol pages; inventory updated;
  every fence passes check-docs; site builds via docgen.
- seed regenerated; `x bootstrap-cfree` fixpoint holds.

Closes #10

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/docgen/inventory.json

@@ -468,5 +468,29 @@
     "unicode-lower",
     "unicode-truncate",
     "unicode-grapheme_len"
+  ],
+  "path": [
+    "path-join",
+    "path-dir",
+    "path-base",
+    "path-ext",
+    "path-stem",
+    "path-normalize"
+  ],
+  "fs": [
+    "fs-exists",
+    "fs-is_dir",
+    "fs-read_text",
+    "fs-write_text",
+    "fs-append_text",
+    "fs-remove",
+    "fs-size",
+    "fs-mkdir",
+    "fs-copy",
+    "fs-list"
+  ],
+  "mime": [
+    "mime-of",
+    "mime-sniff"
   ]
 }