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>

docs/language/path/path-stem.md

@@ -0,0 +1,25 @@
+---
+id: path-stem
+name: Path.stem
+category: path
+kind: namespace-method
+tokens: Path.stem
+sig: Path.stem(p) -> string
+tip: The final component without its extension.
+order: 5
+ns: Path
+member: stem
+---
+
+Returns the final component with its extension removed — the "stem". Useful for deriving an id or a sibling filename. <code>stem("mods/foo/data.json")</code> is <code>"data"</code>.
+
+Parameters:
+- `p` — the path
+
+```ludic
+program Stem {
+  entry {
+    if Path.stem("data.json") == "data" { print(1) }
+  }
+}
+```