fix(journal): read-path non-mutation + CLI hardening + finite createServer defaults

[jpr5]

Summary

Follow-up polish to v1.14.1's journal OOM fix (#114). Three independent improvements surfaced by CR:

1. Read-path non-mutation bug fix. Journal.getFixtureMatchCount(fixture, testId) was a read method that silently inserted an empty Map + triggered FIFO eviction for unknown testIds. Reads could evict live testIds. Now split into a read-only public getFixtureMatchCountsForTest (returns transient empty Map on miss) and private getOrCreateFixtureMatchCountsForTest (insert+evict write path used only by incrementFixtureMatchCount).

2. CLI validation hardening. --journal-max -5 was silently treated as unbounded; now rejected with a clear error. Same for the new --fixture-counts-max flag.

3. createServer() default flip. journalMaxEntries (1000) and fixtureCountsMaxTestIds (500) now default to finite caps for programmatic callers — long-running embedders no longer inherit the original leak. Tests using new Journal() directly remain unbounded by default (back-compat). Opt in to unbounded via journalMaxEntries: 0.

Test plan

• 3 new tests: read-non-mutation, CLI negative rejection, fixtureCountsMaxTestIds cap FIFO eviction
• Full suite: 2461 tests pass
• Lint + build + prettier clean
• CR R2 on combined diff: 0 blocking bugs

Breaking change note

The createServer() default flip is a behavioral change for programmatic embedders that relied on unbounded journal retention. Opt back in with createServer({ journalMaxEntries: 0 }) if needed. Documented in types.ts JSDoc.

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commit: b2df584