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/fs/_section.md

@@ -0,0 +1,11 @@
+---
+id: fs
+title: Fs
+order: 6
+---
+
+The filesystem, wrapped into one safe, ergonomic API — no file descriptors, no byte buffers, no half-written files. The foundation for saves, config, mods, and asset loading: read and write whole files as text, check existence, make directories, copy, and list a folder. The bare <code>file_*</code> builtins remain underneath for streaming; <code>Fs.*</code> is the everyday layer.
+
+Fallible calls surface failure as a value — <code>null</code>, <code>false</code>, or <code>-1</code> — that you branch on, never a crash (a first-class <code>try/else</code> lands with the error-handling work). Writes are atomic (written to a temporary file and renamed into place), so a crash mid-write never corrupts the previous file. <a href="fs-list"><code>list</code></a> returns entries sorted, so a directory walk is reproducible across runs and platforms.
+
+Paths are <code>/</code>-separated over the native (macOS/BSD) filesystem — the fully supported target today; a sandboxed virtual filesystem for wasm and recursive directory copy are follow-ups. Text helpers are UTF-8 (they pair with <a href="ns-Unicode"><code>Unicode</code></a>); use <a href="ns-Path"><code>Path</code></a> to build the paths and <a href="ns-Os"><code>Os</code></a> for per-user locations.