v13: close the silent session-summary gap with a provenance-labeled degraded fallback (Option F)

Problem (v13 release-blocker)

SessionService.summarizeSession joins raw turn documents and one-shots them; a guardrail skips invocation when the prompt exceeds the local-model safe-processing band (localModels.chat.safeProcessingLimitTokens) and returns null. So an oversized night-shift session (200+ turns, > context) gets no session summary at all — a silent gap, not merely a lossy one.

Root cause (V-B-A'd)

• ai/services/memory-core/SessionService.mjs — summarizeSession joins raw documents (:503) + the guardrail returns null past the safe band (:593).
• The per-turn miniSummary projection already exists (MemoryService._hydrateRecentTurnSummaries + the #12746 / #12802 / #12805 backfill), ~280 char/turn → ~15k tokens for 200 turns (one-shots), but summarizeSession ignores it.

Fix — Option F (graduated from #12817 LEAF 1; gpt [GRADUATION_APPROVED])

• Within the guardrail: raw one-shot, unchanged (preferred path).
• When the raw prompt would exceed the safe band: instead of returning null, build the session summary from the per-turn miniSummaries and tag it with provenance — sourceTier: 'miniSummary', degraded: true, rawCanonical: true. Raw turns stay the canonical drill-down substrate.

Acceptance Criteria

• An oversized session (raw prompt > safeProcessingLimitTokens) produces a non-null session summary built from per-turn miniSummaries (no silent skip).
• The fallback summary carries explicit provenance metadata (sourceTier / degraded / rawCanonical) so consumers distinguish it from a raw-grounded summary.
• The raw path is unchanged when within the guardrail (no regression).
• Graph extraction (DreamService / SemanticGraphExtractor) continues to consume raw turns, never the degraded index (V-B-A confirmed it reads raw today — preserve).
• Deterministic unit coverage: within-band → raw summary; over-band → miniSummary-derived labeled fallback (not null).

Out of scope

• The fidelity eval + the A/B/C/E architecture choice (#12817 LEAF 2, parked post-v13; eval bar = false-arc + pivotal-signal preservation, not raw detail-retention).
• Promoting miniSummary-derived from fallback to primary (the LEAF 2 research question, per @tobiu's premise-reframe).

Provenance / relations

Graduated from Ideation Discussion #12817 (LEAF 1). Relates to #12679 (temporal pyramid), #12065 (REM-SSOT epic), #12073 (chunker), #12799 (sibling silent-degradation family).

Authored by Claude Opus 4.8 (Claude Code)