P6.5: Add Mode C verification summary notes

Documents the completed P6.5 Mode C work-queued chunked jobs implementation.
All acceptance tests pass; infrastructure fully functional per plan §14.5.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

notes/miroir-m9q.5.md

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+# P6.5 Mode C: Work-Queued Chunked Jobs - Verification Summary
+
+## Task Completion Status
+
+P6.5 Mode C work-queued chunked jobs (plan §14.5) is **fully implemented** and all acceptance tests pass.
+
+## Implementation Details (from commits 8b1cf42, cff90a3)
+
+### Core Components
+1. **mode_c_coordinator.rs** - Job coordination with:
+   - `claim_job()` - atomic compare-and-swap for job claiming
+   - `renew_claim()` - heartbeat to extend claim TTL
+   - `reclaim_expired_claims()` - release claims from crashed pods
+   - `split_job_into_chunks()` - chunk large jobs on input boundaries
+   - `queue_depth()` - HPA metric support
+
+2. **mode_c_worker/mod.rs** - Worker loop for processing:
+   - Poll for queued jobs and claim them
+   - Heartbeat to renew claims every 10s
+   - Process dump import chunks (NDJSON line boundaries)
+   - Process reshard backfill chunks (shard-id ranges)
+   - Handle idempotent resume from `last_cursor`
+
+3. **dump_chunking.rs** - Split NDJSON dumps on line boundaries (256 MiB default)
+
+4. **reshard_chunking.rs** - Split reshard backfill by shard-id ranges
+
+### Database Schema
+Migration 005_jobs_chunking.sql adds:
+- `parent_job_id` - Link chunks to parent job
+- `chunk_index` - Chunk position (0-based)
+- `total_chunks` - Total number of chunks
+- `created_at` - Job creation timestamp
+- Indexes for efficient queries
+
+### Acceptance Tests (22 tests pass)
+- ✅ 1 GB dump splits into 4× 256 MiB chunks
+- ✅ 3 pods claim chunks in parallel
+- ✅ Claim expires in 30s; another pod resumes at last_cursor
+- ✅ HPA queue depth metric drives scaling
+- ✅ Two concurrent dumps interleave without starvation
+- ✅ Reshard backfill splits by shard-id range
+- ✅ Heartbeat renews claim; missed heartbeat expires
+
+## Configuration
+
+```yaml
+dump_import:
+  chunk_size_bytes: 268435456  # 256 MiB per §14.5 Mode C chunk-parallel coordinator
+```
+
+## HPA Integration
+
+Queue depth metric: `miroir_background_queue_depth` (Prometheus GaugeVec with `job_type` label)
+
+```yaml
+# Example HPA configuration
+metrics:
+- type: External
+  external:
+    metric:
+      name: miroir_background_queue_depth
+    target:
+      type: AverageValue
+      averageValue: 10
+```
+
+## Verified
+
+- All 22 Mode C acceptance tests pass
+- Jobs table with states: `queued | in_progress | completed | failed`
+- Claim TTL: 30s default, heartbeat every 10s
+- Chunking on input boundaries (NDJSON lines for dump, shard-id for reshard)
+- Per-chunk progress for idempotent resume
+- Queue depth metric for HPA scaling