feat: lifetime elision, tooling CI, registry, and LSP locals

Ship the QUALITY_PLAN stretch from ownership through ecosystem: C.1
lifetime elision (bootstrap + selfhost), bux fmt/test/doc CI hooks,
stdlib goldens, package registry (bux search/add), and LSP 0.4
position-sensitive locals with inferred let types. Full-tree format
pass plus Map/Set remove double-free fix.

docs/SEMVER.md

@@ -0,0 +1,76 @@
+# Bux Semantic Versioning Policy
+
+> Status: Draft for v0.x → v1.0 freeze (E.3)
+
+Bux follows [Semantic Versioning 2.0.0](https://semver.org/) with the
+clarifications below.
+
+---
+
+## Version numbers
+
+```
+MAJOR.MINOR.PATCH[-prerelease]
+```
+
+| Component | When it increases |
+|-----------|-------------------|
+| **MAJOR** | Breaking language / stdlib / CLI changes |
+| **MINOR** | Backward-compatible features |
+| **PATCH** | Backward-compatible bug fixes |
+
+During **0.x** (pre-1.0):
+
+- `0.MINOR.PATCH` — MINOR may still introduce breaking changes (documented in
+  the release notes and `MIGRATION_*.sh` when needed).
+- Prefer deprecation warnings for at least one MINOR before removal when
+  practical.
+
+After **1.0.0** (language freeze):
+
+- Breaking changes require a MAJOR bump and a migration guide.
+- The Language Reference is the normative spec; compiler bugs that contradict
+  the ref are fixed without a MAJOR bump.
+
+---
+
+## What counts as “breaking”
+
+- Removing or renaming a public stdlib symbol
+- Changing the type or semantics of a public API
+- Changing CLI flags that scripts rely on (`build`, `test`, `fmt --check`, …)
+- Changing `bux.toml` / `bux.lock` fields in an incompatible way
+- Changing the fat `func` / tuple C ABI in a way that breaks linked code
+
+**Not breaking:**
+
+- New keywords that were previously valid identifiers only if reserved carefully
+  (prefer contextual keywords)
+- New diagnostics / stricter `@[Checked]` (document; may be gated)
+- Formatter whitespace-only changes
+
+---
+
+## Package versions (registry)
+
+Registry packages use the same MAJOR.MINOR.PATCH scheme.
+
+`bux add foo` / `bux add foo 0.1` resolution:
+
+| Request | Matches |
+|---------|---------|
+| `*` / omitted | Latest entry for `foo` in the index |
+| `0.1.1` | Exact version |
+| `0.1` | First version with that prefix (e.g. `0.1.1`) |
+
+Lockfiles pin the **resolved** version and source path/URL.
+
+---
+
+## Release checklist (maintainers)
+
+1. Update `docs/LanguageRef.md` if behaviour changed
+2. Update `docs/QUALITY_PLAN.md` / changelog notes
+3. Run `make test` (includes `fmt-check`, examples, goldens)
+4. Run `make selfhost-loop`
+5. Tag `vMAJOR.MINOR.PATCH`