feat: Phase 09 P0 — production execution closure plan

Execution-closure targets:
- P1: TransferFullBase — reuse rebuild.go TCP protocol
- P2: TransferSnapshot — checkpoint image + WAL tail
- P3: TruncateWAL — AdvanceTail + superblock update
- P4: Runtime ownership — V2 orchestrator drives execution

Key reuse sources identified:
- rebuild.go: rebuildFullExtent (client), RebuildServer (server)
- wal_writer.go: AdvanceTail
- flusher.go: updateSuperblockCheckpoint
- blockvol.go: ScanWALEntries (already wired)

Slice order: full-base first (highest value), then snapshot,
then truncation, then runtime ownership.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sw-block/design/agent_dev_process.md

@@ -101,6 +101,10 @@ Each delivery should include:
 3. resources acquired/released
 4. test inventory
 5. known carry-forward notes
+6. reuse note:
+   - files updated in place
+   - files used as references only
+   - files copied and why
 
 This template is required between:
 
@@ -126,6 +130,9 @@ Test inventory:
 
 Carry-forward notes:
 - ...
+
+Reuse note:
+- ...
 ```
 
 ## Phase Doc Usage
@@ -153,6 +160,10 @@ Use for:
 3. carry-forward discussion
 4. open observations
 5. why wording or scope changed
+6. slice-level reuse instructions:
+   - `update in place`
+   - `reference only`
+   - `copy is allowed`
 
 This document may be longer and more detailed.
 
@@ -290,6 +301,20 @@ Any such reuse should be reviewed explicitly as:
 3. temporary carry-forward
 4. hard gate before later phases
 
+### Rule 6: Every substantial slice should declare reuse instructions
+
+Before implementation grows, the slice package should state:
+
+1. which existing files are expected to be updated in place
+2. which existing files are reference-only
+3. whether any copying is allowed and why
+
+This helps prevent:
+
+1. accidental scope growth
+2. unclear ownership of old files
+3. hidden semantic inheritance from V1/V1.5 paths
+
 ## Current Direction
 
 The project has moved from exploration-heavy work to evidence-first engine work.