Test grep against legacy cache partitioning

[philcunliffe]

Summary

• lock down forced-refresh grep against the cache partition layout shipped before session_id became required
• prove committed legacy rows and newly pending rows remain searchable after the flush
• keep the mitigation in existing runtime code; this PR adds only the regression test

Validation

• focused grep tests: 23 passed
• full npm test: 5,379 passed, 2 skipped
• npm run typecheck
• hyp dev smoke query_grep_roundtrip
• installed hypaware 1.28.0 live forced-refresh grep on local data from 2026-08-26 onward
• installed 1.28.0 isolated pre-session-id table plus pending spool: 2 of 2 hits