release: SemVer adoption + v0.3.0 changelog entry

club-3090 is software (docker composes + system scripts + patch bundles
that downstream rigs run as-is), not just rolling recipes. Switch from
CalVer to SemVer from v0.3.0 onward; past CalVer tags (v2026.05.09,
v2026.05.10) preserved for history.

CHANGELOG.md: convention note + retroactive CalVer→SemVer mapping +
new v0.3.0 (2026-05-10) entry covering 8 commits since v2026.05.10:
- Qwen 3.6 27B thinking OFF default across all 21 composes
- MODEL_DIR UX overhaul (closes #116) — interactive setup prompt,
  $MODEL_DIR placeholder everywhere
- power-cap-sweep --include-commit (closes #112)
- BENCHMARKS aider-polyglot-30 row

cliff.toml: drop "snapshot of the rolling stack — not a versioned
API" framing; add SemVer note. Tag pattern v[0-9]+.[0-9]+.[0-9]+
already matches both CalVer and SemVer, so the cliff release
workflow needs no changes.

cliff.toml

@@ -34,7 +34,7 @@ body = """
 
 ## Pinning to this release
 
-This is a snapshot of the rolling stack — not a versioned API. To pin to this exact state:
+**Versioning:** SemVer in `0.x` — treat any minor bump as potentially breaking until `1.0`. Past CalVer tags (`v2026.05.09`, `v2026.05.10`) are preserved for history; SemVer takes over from `v0.3.0` onward.
 
 ```bash
 git checkout {{ version }}