feat(context,memory): persist ContextFidelity per message for cross-turn floor invariant (#4550)

[bug-ops]

Summary

CAM Phase 2-B: fidelity levels are now persisted to SQLite after each scoring pass and reloaded on the next turn, enforcing the floor invariant across conversation turns.

• Migration 095 (zeph-db): additive ALTER TABLE messages ADD COLUMN fidelity_tag INTEGER NOT NULL DEFAULT 0 (SQLite ≥ 3.37 O(1); PostgreSQL SMALLINT NOT NULL DEFAULT 0)
• ContextFidelity::from_u8 (zeph-common): converts DB integer → enum; unknown values → Full (safe default); 0 → None sentinel avoids false floor on pre-CAM rows
• MessageStore::update_fidelity_tags (zeph-memory): batch-writes (MessageId, u8) pairs in a single transaction; no-op when slice is empty; load_history / load_history_filtered now populate fidelity_tag from DB
• Floor invariant (zeph-context): score_and_apply gains allow_upgrade: bool — false enforces floor (Compressed cannot upgrade to Full; Placeholder cannot upgrade at all); true bypasses floor on the proactive regrade path; more_restrictive() propagates floor through tool-use / tool-result atomicity pairs
• persist_fidelity_tags (zeph-agent-context): private helper that batch-writes after normal scoring and after regrade path, so floor constraints survive to the next turn
• truncate_to_tokens (zeph-context): manual is_char_boundary backward scan replaced with str::floor_char_boundary (stable since Rust 1.91)

Test plan

• 4 new round-trip tests in zeph-memory (update_fidelity_tags_round_trip, placeholder_round_trip, zero_maps_to_none, empty_is_noop) — all pass
• 8 new floor invariant tests + 1 mixed atomicity test in zeph-context — all pass
• 1766 tests across zeph-common, zeph-context, zeph-memory pass
• clippy clean on all touched crates
• fmt clean workspace-wide
• No blocking std::fs — all DB writes are async

Closes #4550