test(e2e): gate heavy GPU serves to the merge queue

[fredespi]

Summary

The full real-GPU serve matrix ran on every PR, making the per-PR GPU lanes slow. This gates the redundant heavy serves to the merge queue while keeping one real serve per engine on the PR path as a pre-merge canary, so a broken serve is still caught before a change enters the queue.

Changes

• Add a @merge-queue scenario axis, mirroring the existing @nightly one: a new merge_queue field on ScenarioDecl, an include_merge_queue arg to resolve() with a skip branch, and an E2E_MERGE_QUEUE env read in the harness.
• Tag the redundant default-engine and readiness serves (serve-default-engine-working-endpoint, serve-default-engine-inference, serve-readiness-contract) @merge-queue so they run only in the merge queue.
• Keep serve-vllm-inference (vLLM) and serve-lemonade-inference (lemonade) untagged as per-engine PR canaries.
• ci.yml sets E2E_MERGE_QUEUE on the merge_group event for all three GPU jobs. The jobs still run on pull_request and produce their required checks; only the work is trimmed.

Test plan

• cargo fmt --check, cargo clippy --workspace --all-targets --exclude e2e-cucumber -- -D warnings clean (Linux container).
• cargo test --workspace + cargo test -p e2e-cucumber --lib green (64/64, incl. a new merge_queue_scenario_skips_unless_included unit test).
• E2E mock lane reconciliation: 3 xfail / 0 XPASS / 0 unexpected.
• Scoped MI300X dispatch verified the grid on hardware: scenarios 6/6b/8 resolve to skip on the PR path, canaries 5/7 run and serve; 0 unexpected failures.

[rominf]

Automated review pass over the diff against main. No code changed. Verdict: approve with minor changes — one doc fix is worth landing before merge; the rest are judgement calls for you.

The mechanism itself checks out. Verified directly: the @merge-queue axis mirrors @nightly/@lifecycle exactly, the gate is evaluated cheapest-first (before the xfail matrix lookup), the empty-string env value is handled correctly by is_some_and(|v| v == "1"), merge_group: is a real trigger and the merge queue is active on main, the changes job forces heavy=true off-PR so the GPU jobs really do run in the queue, and cargo test -p e2e-cucumber --lib is green locally (56/56). tests/e2e-cucumber/tests/e2e.rs:828 is the only non-test call site of resolve().

Findings, most severe first

1. tests/e2e-cucumber/README.md:97-106 — stale tag table (worth fixing before merge).
The component's own tag-axis table lists @id, @requires-gpu, @requires-engine, @requires-os, @serve-timeout, @nightly, @lifecycle. This PR adds a peer axis without adding the row. Failure scenario: a contributor sees scenarios 6/6b/8 skipped on their GPU dispatch, finds no @merge-queue in the documented tag list, and either re-tags them or "fixes" the skip. (@requires-no-gpu is also missing — pre-existing, cheap to add while you're there.)

2. .github/workflows/nightly.yml:346,430,525 — nightly GPU lanes never set E2E_MERGE_QUEUE (advisory).
All three nightly GPU jobs set E2E_INCLUDE_NIGHTLY: "1" unconditionally but did not get the new var, so the heavy serves now run only on the merge_group event. The merge queue is confirmed active, so this is not a coverage hole today — it is a redundancy question. Failure scenario: if the merge-queue path is ever bypassed or broken, scenarios 6/6b/8 run nowhere and the nightly grid shows three quiet skip cells instead of surfacing the gap. One line per job restores the belt-and-braces; if omitting it is deliberate, a sentence in the PR body would make that clear.

3. .github/workflows/ci.yml:759-764 — the comment says "Opt-in @merge-queue serves", but there is no way to opt in.
E2E_INCLUDE_NIGHTLY has a workflow_dispatch input (include_nightly, ci.yml:41-44) precisely so a scoped dispatch can probe one heavy scenario. @merge-queue has no equivalent. Failure scenario: scenario 8 regresses in the queue and there is no way to reproduce it on hardware via dispatch — you have to push a branch and get it queued, or hand-edit the workflow. Mirroring include_nightly with an include_merge_queue boolean input would keep the two axes symmetric and make the comment true.

4. The PR description's framing overstates the safety net (pre-existing mechanism, but this change leans on it).
All three GPU e2e jobs are continue-on-error: true, and xtask e2e-report returns Ok(()) regardless of unexpected failures (xtask/src/e2e_report.rs:47-59) — while E2E tests (GPU), E2E tests (Strix Halo, Ubuntu), E2E tests (Strix Halo, Windows) and E2E consolidated report are all required contexts on main (verified via the branch-protection API). So a failing serve reports green and blocks nothing, on either path. Failure scenario: "a broken serve is still caught before a change enters the queue" reads as gating, but both paths are advisory, and the merge-queue results land in a run nobody looks at once the PR has merged. Not introduced here — but the rationale would be more accurate as "advisory signal on the PR, full advisory matrix in the queue".

5. tests/e2e-cucumber/src/expectation.rs:339-346 — resolve() now takes three adjacent bare bools.
include_nightly, include_lifecycle, include_merge_queue are positionally interchangeable and a transposition compiles silently; roughly 35 unit-test call sites pass bare literals. Failure scenario: a future fourth axis is inserted at the wrong position, or a test literal is swapped, and merge_queue_scenario_skips_unless_included — which exists specifically to prove the nightly and merge-queue axes are independent — asserts the wrong thing while still passing. The codebase already prefers named-options structs for multi-flag inputs (RunnerOptions, FixOptions, ComfyUiInstallOptions, DemoOptions). Cross-PR note: #155, #156 and #157 are each independently growing this trailing-bool list — a shared ScenarioSelection/RunMode struct is better done once across the three than three times.

6. tests/e2e-cucumber/src/expectation.rs:359 — skip reason is imprecise.
"merge-queue-only scenario; set E2E_MERGE_QUEUE to run", but the harness requires the value to be exactly "1" (tests/e2e-cucumber/tests/e2e.rs:756); the sibling lifecycle message correctly says set E2E_INCLUDE_LIFECYCLE=1 to run. Failure scenario: someone exports E2E_MERGE_QUEUE=true, the scenario still skips, and the message they just followed told them they did it right.

Checked and clean

• xfail reconciliation: scenarios 6/6b/8 do have live xfail rows in expectations.toml, but resolve() returns Skip before the matrix lookup, so CellOutcome::Skip and is_problem() == false (crates/e2e-report/src/lib.rs:659-684). No spurious stale-xfail or XPASS. The advisory xfail-hint script is text-only and unaffected.
• merge_group path filtering: paths-filter is pull_request-only (ci.yml:92) with forced=true off-PR (ci.yml:150-153), so no base-ref ambiguity and no docs-only skip in the queue.

Worth reinforcing

• Placing the merge-queue gate with the other applicability gates, ahead of the matrix, is the right ordering, and it is explicitly tested for cheapest-first behavior on a no-GPU host.
• Keeping one untagged real serve per engine rather than dropping all GPU serves from the PR path is a good cost/coverage trade, and the feature-file comments explain the canary role right where a future editor will read it.
• The new unit test asserts axis independence (nightly on, merge-queue still skipped), which is the non-obvious property worth pinning.

Not verified

• The GPU suite was not run here; the hardware-dispatch evidence in the description is taken at face value.