feat(dag): run unordered workspace writers in parallel with an implementation aggregator

[LeXwDeX]

Closes #293

Summary

The block compiler chained every unordered coding/prototype writer into one serial lane, so saved routes advertised parallel implementation slices that never overlapped — the runtime concurrency budget bought nothing for the phase under the most time pressure. Root cause of the old serialization: the diff review gate binds one implementation reference and one fingerprint, and parallel writers have no single canonical evidence source.

Change (see ADR-0002, included)

• Serialization removed; writer ordering exists only where the author declared it.
• When an implementation review covers writers with no total order, the compiler injects one read-only <review>--aggregate node between the writers and the verification gate: it receives each writer's declared changed_files, fails loudly on any write-set intersection, and otherwise publishes the union plus one fingerprint computed at the convergence point.
• The verify block is rewired onto the aggregator and receives the implementation fingerprint binding (fixes the verify contract's empty promise).
• Total-ordered writer chains compile byte-identically (canonical-writer behavior preserved).

Runtime, review lifecycle, settlement, and recovery are untouched; the aggregator is an ordinary durable node with the ordinary implementation schema.

Verification

• 3 existing tests that pinned forced serialization rewritten to the new contract; new test/dag/blocks-parallel-writers.test.ts covers the issue's acceptance list (aggregator shape, partial order, chain byte-identity, collision, rewire preservation, verify binding).
• 558 DAG/authoring tests green; bun typecheck clean from packages/opencode and packages/core.
• Guide wording updated in workflow-blocks.md (triple-disjoint write-set criterion).

Follow-ups (separate repos/PRs)

• opencode-dag-config#feat/parallel-writers-contract (prepared, commit 20698f3) lands after this merge; its runtime-compat.json bump keys off this merge SHA (incompatible block rule).