fix(brain-cache): normalize valid-but-partial _meta.json so consumers don't crash

[jbetala7]

Problem

bin/gstack-brain-cache crashes with a TypeError when its _meta.json is valid JSON but lacks the last_refresh map. loadMeta already starts fresh for a missing file and for corrupt (unparseable) JSON, but a file that parses successfully is returned verbatim with no normalization. Three consumers then dereference meta.last_refresh unguarded:

• isStale (reached via cmdGet's warm-path check) — meta.last_refresh[entityName]
• cmdInvalidate — delete meta.last_refresh[entityName]
• refreshEntity — meta.last_refresh[entityName] = Date.now()

refreshEntity already guards the sibling map (meta.last_attempt = meta.last_attempt || {}), and the CacheMeta type marks last_attempt? optional while last_refresh is required — so the guard was applied to only one of the two maps.

Repro (on main)

A _meta.json whose schema_version and endpoint_hash match the current pack (so the schema/endpoint rebuild path is not taken) but with no last_refresh key:

• cmdGet('product', 'helsinki') → TypeError: undefined is not an object (evaluating 'meta.last_refresh[entityName]')
• cmdInvalidate('product', 'helsinki') → TypeError: undefined is not an object (evaluating 'delete meta.last_refresh[entityName]')

Such a meta can arise from external tooling, a hand-edit, or any valid-but-partial persisted state — the same class of input the corrupt-JSON branch already guards against.

Root cause

loadMeta normalizes the missing-file and corrupt-JSON cases but returns a valid-but-partial meta as-is, leaving last_refresh/last_attempt possibly undefined for downstream consumers.

Fix

Normalize both maps once in loadMeta so a partial file degrades like the corrupt-file case instead of crashing. A meta without last_refresh is now treated as never-refreshed: cmdGet falls through to a normal cold-refresh/stale path and cmdInvalidate is a safe no-op.

Testing

• bun test test/brain-cache-roundtrip.test.ts test/brain-cache-spec.test.ts → 32 pass / 0 fail
• Added two regression tests (cmdGet/cmdInvalidate against a _meta.json lacking last_refresh); both fail on main with the TypeError above and pass with this change.

Fixes #1879

🤖 Generated with Claude Code

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