fix(catalog): version the .cache wire format so parser changes auto-invalidate

[ale-rinaldi]

Closes #5.

What changed

pkg/catalog/cache.go wraps the on-disk gob payload with a tiny header:

type cacheFile struct {
    Magic         string  // "mythy-catalog-cache"
    SchemaVersion int     // parserSchemaVersion
    Template      *Template
}

SaveCache always emits the wrapper. LoadCache verifies the magic and version before unwrapping; any mismatch routes to ErrCacheStale. The pre-existing source-XML-mtime check is kept — it still catches the "I edited the catalog by hand" case independently of the schema version.

Bumping policy

parserSchemaVersion is a constant at the top of cache.go, currently 1. The doc comment spells out the rule: any PR that changes how pkg/catalog/ encodes a Template (struct field added, dropped, semantics-rewritten, or any load-time pass mutates the result) bumps the version. Reviewers should treat unbumped pkg/catalog/ diffs as suspicious.

Backwards compatibility

Pre-versioning caches encoded a bare *Template directly. After this PR LoadCache tries to decode them into the new cacheFile wrapper and gets a gob error or a malformed wrapper; either way it returns ErrCacheStale and the caller regenerates. Verified by TestLoadCacheRejectsLegacyUnwrappedFormat.

Tests

• TestLoadCacheRejectsLegacyUnwrappedFormat — writes a bare *Template (the pre-fix format), asserts LoadCache returns ErrCacheStale.
• TestLoadCacheRejectsWrongSchemaVersion — writes a wrapper with parserSchemaVersion + 1, asserts the same.
• Existing TestCacheRoundTrip / TestCacheStaleOnMtime / TestLoadCacheResolvesTypedefsForStaleCaches pass unchanged through the new wrapper.

Live verification

Pre-PR cache on disk: ~/gridsociety/thytronic-templates/us/NV10P-EB0-u.cache (1931115 bytes, mtime 13:13, written by an older mythy build). After running any mythy command with this PR's binary, the cache regenerates: same path, mtime advanced, size grew by 91 bytes (the wrapper header), and subsequent invocations load the new-format cache without re-parsing. The migration is silent.

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