feat(#3034): add opt-in parallel reviewer lanes (#3822)

* test(#3034): failing-first coverage for opt-in parallel reviewer lanes

Executes the real invoke_reviewers dispatch block from review.md against a
stubbed gsd_run seam rather than pattern-matching the workflow text, so the
two properties that actually carry risk are observable: that every lane is
joined before aggregation, and that concurrent lanes cannot tear a line in
gsd-review-lane-results.jsonl.

Concurrency is proven by a barrier fixture, not by elapsed time -- each stub
lane blocks until all lanes have checked in, which can only complete if they
overlap.

Red against the current sequential dispatch, by design.

Refs #3034

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* feat(#3034): add opt-in parallel reviewer lanes

Reviewer lanes within one review pass inspect the same immutable plan
snapshot and have no dependency on one another, but were dispatched strictly
one at a time, so a multi-reviewer pass cost roughly the sum of its lanes.
The serialization is a deliberate protection against provider rate limits,
so it stays the default; review.parallel_lanes opts a project out of it.

The loop body is hoisted into run_review_lane so the sequential and
concurrent paths share one body -- two hand-synced dispatch bodies is the
divergence class ADR-2782 spent a phase deleting. Each lane writes a
slug-scoped result file, concatenated in selection order after the join:
concurrent O_APPEND is atomic only below PIPE_BUF, and write_reviews parses
that JSONL to render the models:/model_sources: frontmatter, so a torn line
is a broken REVIEWS.md rather than a cosmetic log defect. Aggregating in
selection order also keeps the artifact byte-identical between the two paths.

The guard is strict equality on "true" and falls back to sequential when
config-get fails -- the opposite polarity from the commit_docs guard,
because failing open here fires the very requests the default prevents.

Also corrects docs/COMMANDS.md and its four locale mirrors, which described
--all as running every configured reviewer in parallel when dispatch was in
fact sequential.

Closes #3034

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* fix(#3034): de-duplicate dispatch slugs and scope lane locals

Review finding (Standards axis): a slug repeated in SELECTED_REVIEWERS would
put two concurrent background jobs on the same > -truncated per-lane result
file. The shared-append form this replaced could not corrupt itself that way,
so de-duplicating is what keeps the concurrent path no worse than the
sequential one.

Selection de-dupes today -- the roster is a Set and review.default_reviewers
normalizes lowercase-unique -- but reachability analysis is not a contract,
which is the same reason the roster derivation itself is guarded.

Splitting once into DISPATCH_SLUGS also removes the duplicated tr-split the
same review flagged: the dispatch and aggregation loops now share one list,
which is what guarantees they walk the same slugs in the same order. A plain
string accumulator rather than an array, because zsh and bash disagree on
array indexing and this block runs under both.

Also scopes run_review_lane's locals. Not a live fix -- each dispatched call
already forks its own subshell -- but it makes the isolation a property of the
function rather than of the dispatch mechanism happening to fork.

Refs #3034

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* test(#3034): acknowledge review.md growth, drop spent 2295 ack

The differential attribution gate reported review.md growing 4173 bytes
(30712 -> 34885) with no live acknowledgment. Adds the per-PR fragment it
asks for, naming only the one path it reported.

Deleting tests/emitted-drift-acks/2295-resolved-model.json is required, not
opportunistic. That fragment declared review.md and nothing else, and its
ripple is already absorbed into the base, so it is spent -- it can no longer
clear anything, which is why the gate still reported review.md as
unacknowledged. It could not simply be left alone either: two ack sources may
never name the same path, so it blocked this PR's fragment outright.
CONTRIBUTING is explicit that a fragment whose last entry is removed gets
deleted with it, because an empty fragment signals nothing while its presence
reads as a live alarm.

Refs #3034

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* chore(#3034): backfill changeset PR number

Refs #3034

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

---------

Co-authored-by: sim <sim@local>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

.changeset/zesty-sloths-sprint.md

@@ -0,0 +1,5 @@
+---
+type: Added
+pr: 3822
+---
+**`/gsd-review` can now dispatch reviewer lanes concurrently** — a multi-reviewer pass cost roughly the sum of its lanes even though every lane inspects the same immutable plan snapshot and none depends on another. Set `review.parallel_lanes` to `true` to overlap them within a single pass; the default stays sequential and keeps the provider-rate-limit protection, and convergence cycles stay sequential either way. This also corrects `docs/COMMANDS.md`, which described `--all` as running every configured reviewer in parallel when dispatch was in fact sequential. (#3034)