chore(benchmarks): add 5000-route production-build stress scenario

[hyf0]

No dependencies (standalone benchmark infra). ⚠️ Needs a small perf.yml wiring change (see the bottom of this description) before the scenario runs in CI.

What

• A vinext-only perf scenario "Production build time (5000 routes)" that builds a generated 5000-route app, via a separate benchmarks/vinext-large fixture.
• generate-app.mjs gains --extra <n> / --target <name> (and the VINEXT_BENCH_EXTRA_ROUTES env) to scale and target the generated app.

Why

The default benchmark app is ~33 routes. At that size, super-linear build-time effects — e.g. route-graph directory scanning that re-reads shared ancestor directories once per descendant route — are smaller than run-to-run noise, so regressions (or fixes) in them are invisible to the perf bot. 5000 is the smallest scale where such effects clearly clear the ±1.5% band while keeping CI cost moderate (a scaling sweep showed the relevant delta going ~noise at 1k → ~−32% at 5k → ~−52% at 10k, with signal flattening past ~5–8k).

Notes

• vinext-only and a separate fixture, so the headline vinext-vs-Next comparison and the existing scenarios are unchanged.
• Adds benchmarks/vinext-large to the workspace (lockfile updated).
• The generate step is intentionally always-run (no implementationId), so the 5000-route app is generated even on PR runs that skip Next.js via --implementation=vinext.

⚠️ CI wiring needed (for a maintainer)

This scenario builds a new project (benchmarks/vinext-large) under the isolated vinext-perf-head user, which perf.yml doesn't yet know about. I've left the perf.yml change to whoever owns the perf workflow, since it touches the isolated-user ACL model and I couldn't exercise that path locally. To run green it needs the same treatment benchmarks/vinext already gets:

• grant_write_paths — add benchmarks/vinext-large to project_roots, and benchmarks/vinext-large/{dist,.vite,node_modules/.vite,node_modules/.vite-temp} to paths. Without the write-ACL, vp build (run as vinext-perf-head) hits EACCES writing dist.
• "Validate performance workspace paths" — add benchmarks/vinext-large (+ its app/dist) to the directory / symlink checks, for parity.

No change is needed to the trusted-manifest/harness-copy steps (they only gate vinext/nextjs): the build reads the fixture from the target checkout and its scripts from the trusted harness.

[pkg-pr-new]

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commit: 3cb128e