docs(adr): record the single local-batch intake in ADR-SYNC-001

[winlp4ever]

What

S7c — the docs half of the sync-spine work. Amends ADR-SYNC-001 to record the "single local-batch intake" refinement introduced by #259 (S7a) and #260 (S7b). Docs only, no code.

Changes to the ADR

• Decision gains a MUST rule: all locally-produced batches enter the coordinator through the single submitLocalBatch intake, never a direct oplog write. One intake, two producers, one seq authority — the store scene (attachSync.sendBatch) and any headless / off-scene producer (getBoardSyncRef()) both funnel here. A headless producer MUST pass scene: false, so its off-layer ops never join the rebase set (else applyRemote's undo/replay injects them into the current scene), and MUST record to the oplog before submitting.
• Consequences gains the desync hazard: a direct persistence.record without entering the intake lands in the oplog but triggers no pump, so it ships only opportunistically and isn't serverSeq-stamped promptly. The deep-layer cascade delete (graph/subtree.ts) was the last such bypass, now fixed in fix(sync): pump deep-layer cascade deletes through the sync intake #260.
• Applies to adds graph/subtree.ts; Verify adds a grep for submitLocalBatch (confirmed to resolve to exactly the two producers).
• Status line records the amendment date.

Notes

• The ADR README index row summarizes the unchanged headline decision, so it needs no edit.
• Independent of fix(sync): pump deep-layer cascade deletes through the sync intake #260 (different file); merges in any order. The ADR describes code that lands with fix(sync): pump deep-layer cascade deletes through the sync intake #260.