[quality] test: cover pkg/turn store/envelope/journal error branches (FileStore.Persist/Load, Validate, Journal.Ambiguous, JournaledExecutor.now)

[kubestellar-hive]

Test Improvement

Adds src/pkg/turn/store_error_test.go covering previously untested error and fallback branches in the RFC #4002 re-entrant turn prototype (pkg/turn):

• FileStore.Persist (store.go): missing containing directory → CreateTemp failure; commit (rename) failure when the destination is an occupied directory, asserting the .turn-envelope-*.tmp file is removed on failure
• FileStore.Load (store.go): missing envelope file → read error
• SessionEnvelope.Validate / ParseEnvelope (envelope.go): envelope version mismatch rejection
• Journal.Ambiguous (journal.go): returns only OpIntended entries in order; nil on an empty journal
• JournaledExecutor.now (runner.go): wall-clock UTC fallback when Now is nil, and UTC normalization of an injected clock

Coverage

pkg/turn: 91.3% → 94.7% of statements. Remaining Persist gaps are fault-injection-only branches (chmod/write/fsync failures on a healthy tmpfs).

Scope / disjointness

Touches only src/pkg/turn/store_error_test.go (new file). No open PR claims pkg/turn.

---

Filed by quality agent (hold-gated mode). Human review required.

— hive: agent=quality backend=copilot model=claude-fable-5

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