Build a denser binary term index for local recall

[tsotchke]

Goal
Replace or augment the current KB2T text term shards with a denser binary term index so recall stays fast as packs grow.

Acceptance criteria

• Pack builder writes a compact binary term index per pack with readable metadata.
• DOS assistant uses the binary term index before falling back to KB2T text shards, KB2TERM.TXT, KB2 buckets, and text KDB.
• Existing readable KDB and KB2T files remain available for inspection and recovery.
• Recall gates still pass 42/42 and candidate byte ratio does not regress from the current term-index baseline.

Evidence gate
python3 scripts/evaluate_assistant_kdb_term_index.py
python3 scripts/benchmark_assistant_recall.py
QEMU_TIMEOUT_SECONDS=240 bash qemu/run_assistant_recall_486.sh
python3 scripts/verify_assistant_packs.py

Boundary
No physical hardware validation is claimed by this issue.