store: scope --date thread orphan dump to the requested day

[anish749]

Problem

pigeon read --date X returned data even when the conversation had no date file for X. interleaveThreads always dumped every thread file whose parent wasn't in the selection, leaking parents and replies from unrelated days into a single-day read.

Solution

Pass the requested date through to interleaveThreads. When set, the orphan-dump fallback drops the cross-date parent and keeps only replies whose ts falls on that day. --since and the default window still get the unbounded fallback (their existing post-filter trims by ts).

Tests

• TestReadConversation_Date_NoFile_DoesNotDumpThreads — new regression: thread file from another day must not surface for --date X when X has no date file.
• Existing TestInterleaveThreads_ParentNotInDateFiles still passes (cross-date reply on requested day still surfaces).
• Existing --since orphan tests unchanged.

[anish749]

Superseded by #363 — the right fix is to rebuild the read pipeline around thread completion (filter → complete → dedupe → sort → interleave), not patch the orphan-dump path.