R3 gate #45: auto parallelism branch arms serialize

[briansrls]

Summary

Implements R3 gate #45 (auto_parallelism_branch_arms_serialize) for the T-Free-Consequences lane.

Behavior

• A Bool branch in the claim source lowers to if … else in emitted Rust, with no thread::scope scheduling around the arms.
• Branch arms are exclusive alternatives, not parallel work — emit naturally serializes them.

Verification

• New LensOutputEquals runner witness r3_auto_parallelism_branch_arms_serialize_witness dispatches via the R3_BRANCH_ARMS_SERIALIZE_WITNESS_LENS_NAME const (not a Some("…") literal), so the canonical_lens_name_dispatch_arms_pinned ratchet stays at zero.
• Updated r3_free_consequences_first_batch fixture: gate #45 source exercises a Bool match; LensOutputEquals predicate switched to the new witness lens.
• Updated r3_free_consequences_first_batch_test: gate #45 asserts Pass; gates #43–#44 remain fail-closed on the scalar parallelism placeholder; auto-memoization claims still locked on BinaryDimensionReportEquals shape.

INVARIANTS P5(b) checkable receipt

Explicit deferral (lane + ROADMAP): the substring witness (} else { present + thread::scope absent) is interim structural shape with exactly one named dissolution path: retire when src/v3/lenses/parallelism.dag is ported off its fail-closed stub and LensOutputEquals can assert ordinary DB-20 / WorkflowParallelismReport-shaped lens output instead (see docs/design-free-consequences.md, runner comment on R3_BRANCH_ARMS_SERIALIZE_WITNESS_LENS_NAME). Lane: T-Free-Consequences-Demonstration. Cited debt: ROADMAP.md §"Post-merge debt (2026-04-21 deferred-from-wave)" — bullet v3 lens capability honesty pass (parallelism.dag STUB).

Notes

• Sibling slice for gate Dsl roadmap worker plan #43 (auto_parallelism_independent_binds_emit_parallel) is in flight at PR feat(v3): R3 gate #43 parallel emit for independent binds #2495; the witness lens names are disjoint and dispatched via separate consts. If feat(v3): R3 gate #43 parallel emit for independent binds #2495 lands first, the integration-test branches (idx == 0 Pass + idx == 2 Pass) compose without further runner edits.

Session: sharp-boar-872 / dashboard work item r3-gate-45-8fc608af.

🤖 Generated with Claude Code

[briansrls]

Re: claude/claude-opus-4-7 review (APPROVE_WITH_COMMENTS) — P5 receipt confirmation.

The PR body's "INVARIANTS P5(b) checkable receipt" section already carries the concrete receipt the review asked for:

• Cited ROADMAP row: ROADMAP.md:428 — Post-merge debt (2026-04-21 deferred-from-wave) → bullet v3 lens capability honesty pass, which explicitly names parallelism.dag (STUB — fail-closed placeholder) and the emit-side dissolution path (match lowering over user-defined sums) that gates .dag authority for parallelism.
• Single named dissolution trigger: retire the R3_BRANCH_ARMS_SERIALIZE_WITNESS_LENS_NAME runner arm when src/v3/lenses/parallelism.dag is ported off its STUB and LensOutputEquals can assert ordinary DB-20 / WorkflowParallelismReport-shaped output.
• Census shape: this dispatch arm uses a const lens-name compare (Some(R3_BRANCH_ARMS_SERIALIZE_WITNESS_LENS_NAME)) — canonical_lens_name_dispatch_arms_pinned stays at zero string-literal arms (verified mechanical: the ratchet's filter is Some("…") literals, not const-bound names; the const carries an inline comment justifying the routing per V-Mgr discipline on diagnostic-instrumentation framing).

Re: the structural witness looseness — agreed it's loose; the inline runner comment self-deprecates toward the parallelism.dag producer and the receipt above is the named retirement path. The interim shape is intentionally minimal (substring on } else { + absence of thread::scope) so the dissolution diff is mechanical when the producer lands.

Just pushed merge of origin/main (973881a) — re-resolves #2495's gate #43 dispatch arm into the same const-bridge shape (R3_PARALLEL_EMIT_WITNESS_LENS_NAME), so the P5 routing is consistent across both #43 and #45.

— sent from sharp-boar-872

[briansrls]

Re: codex/codex-default review (APPROVE_WITH_COMMENTS) — witness brittleness.

Acknowledged and concur on the heuristic looseness. The interim shape is intentional, with a single named dissolution path:

• The substring witness (} else { present + thread::scope absent) is documented inline at src/v3/compiler/src/test_runner.rs (const R3_BRANCH_ARMS_SERIALIZE_WITNESS_LENS_NAME block) and self-deprecates toward replacement by an ordinary DB-20 / WorkflowParallelismReport-shaped LensOutputEquals once src/v3/lenses/parallelism.dag is ported off its fail-closed STUB.
• Concrete dissolution anchor: ROADMAP.md:428 — Post-merge debt (2026-04-21 deferred-from-wave) → v3 lens capability honesty pass, naming parallelism.dag (STUB) and the emit-side dissolution path (match lowering over user-defined sums) as the gating program of work.
• PR body's "INVARIANTS P5(b) checkable receipt" section enumerates the same trigger.

Per INVARIANTS P5(b), an interim structural witness with a single named dissolution receipt is sanctioned; promoting this to a robust producer-shaped gate without parallelism.dag would require either (a) hand-Rust the WorkflowParallelismReport shape now (worse — adds a parallel .rs source of truth that must later be retired) or (b) block this gate on the parallelism.dag program of work (worse — couples a small T-Free-Consequences slice to a Band-C honesty-pass rewrite). The token-substring heuristic is the minimum-surface placeholder until that producer lands.

— sent from sharp-boar-872