feat(release): SemVer + ludicc --version, changesets, and x release

The project had no versioning discipline: 0 tags, no CHANGELOG, no way for the
compiler to report a version. Add a lightweight, native release flow.

- Versioning: SemVer, with VERSION as the single source of truth. `ludicc
  --version` (and `ludic --version`) read it at runtime — so a bump touches one
  file and never reseeds the compiler. `x version` reports it too.
- Changesets: one small Markdown file per user-facing change under changes/
  (bump level + type + summary; see changes/README.md). This replaces "remember
  to edit the changelog" with a mergeable artifact, no Node changeset tool.
- `x release [major|minor|patch] [--publish]`: fold the pending changesets into a
  new CHANGELOG.md section (grouped by type), bump VERSION, commit, and tag
  vX.Y.Z. The level defaults to the highest changeset bump. `--publish` also
  pushes and creates the Forgejo release with source + toolchain tarballs;
  tools/ci/forgejo_release.py is the small stdlib-Python HTTP glue for the
  release API (a native Http client is issue #6).

Seed the initial changesets describing the shipped surface; the first `x release`
turns them into the v0.1.0 CHANGELOG. Reseeded for the --version flag; C-free
bootstrap fixpoint holds; suites 56 / 29 / 29 on macOS, 51 / 28 (+skips) on Linux
CI, bootstrap-cfree byte-identical on both.

Part of the repository-cleanup / DX pass (with #32, #34).

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

CONTRIBUTING.md

@@ -63,6 +63,27 @@ The stdlib lives in the runtime (`runtime/`) and is surfaced as namespaces
 3. Add or extend an example under `examples/` and a case in the test suite.
 4. Run `python3 tools/docgen/gen.py && python3 tools/docgen/check.py` — the
    check fails if any inventory symbol lacks a page or is still seed text.
+5. Add a **changeset** for the user-facing change: a small file under
+   [`changes/`](changes/README.md) with a `bump:` level and a one-line summary.
+   The next release folds it into `CHANGELOG.md`.
+
+## Versioning & releases
+
+The toolchain is versioned with [SemVer](https://semver.org); `VERSION` is the
+single source of truth and `ludicc --version` (or `x version`) reports it.
+
+Releases are changeset-driven. Every user-facing change ships with a changeset
+(step 5 above). To cut a release:
+
+```bash
+x release [major|minor|patch]   # omit the level to derive it from the changesets
+```
+
+That aggregates the pending changesets into a new `CHANGELOG.md` section, bumps
+`VERSION`, commits `chore(release): vX.Y.Z`, and tags it. Add `--publish` (with
+`FORGEJO_TOKEN` set) to also push and create the Forgejo release with source and
+toolchain tarballs. The tag doubles as the reproducible bootstrap point: the
+source archive plus its checked-in seed rebuild that exact toolchain.
 
 ## Conventions