…logs, stress fixtures

Prototype series measured only the OTLP traces fixture plus a 3-field
SimpleMessage. Phase 2 of the investigation needs performance readings
across the payload classes protobuf-es actually sees in production, so
that a regression (or improvement) can be attributed to a class of
workload rather than lumped into a single OTLP traces number.

New .proto fixtures under benchmarks/proto/:

- otel-metrics.proto — OTLP metrics export: Gauge/Sum/Histogram oneof,
  NumberDataPoint, HistogramDataPoint with explicit bucket bounds.
- otel-logs.proto — OTLP logs export: LogRecord batch with severity,
  string body, trace/span correlation IDs.
- k8s-pod.proto — Kubernetes Pod list subset: ObjectMeta with
  labels/annotations maps, containers with env/ports/resources,
  container statuses. Map-dominant config payload.
- graphql.proto — GraphQL request/response envelope: long query string,
  map<string,bytes> variables, JSON-in-bytes data + structured errors
  with paths.
- rpc-simple.proto — baseline RPC envelope: routing fields, small
  headers map, opaque bytes payload. Lower bound on per-call overhead.
- stress.proto — synthetic payload: depth-8 self-nesting + 200-wide
  scalar/string/message arrays + 4 KB blob + every proto3 scalar type
  exactly once. Surfaces type-specific regressions that OTLP-only
  fixtures don't hit.

New runner in benchmarks/src/bench-matrix.ts:

- Runs toBinary + fromBinary across all 10 fixtures (including the
  existing SimpleMessage and ExportTraceRequest).
- Emits the standard tinybench table plus a machine-readable JSON
  summary on stdout so CI can diff runs without scraping ANSI output.

Extends benchmarks/src/fixtures.ts with realistic builders for each
shape. Scale knobs (METRICS_SERIES_COUNT, LOGS_RECORD_COUNT,
K8S_POD_COUNT, STRESS_DEPTH, STRESS_ARRAY_WIDTH) are exported so a
follow-up pass can parameterize across multiple sizes without editing
the fixture code.

No changes to existing bench-*.ts files or to the protobuf-es runtime.
The matrix runner is opt-in via `npm run bench:matrix` and the existing
`npm run bench` aggregate is unchanged.

Smoke run confirms every fixture encodes and decodes successfully on
Node v25.8.1 / linux/x64. Encoded sizes span 19 B (SimpleMessage) to
35 KB (OTLP traces).

- Apply biome format fixes to benchmarks and protobuf files
- Prepend `mkdir -p src/gen-protobufjs` to generate:protobufjs so the
  output directory exists on fresh CI checkouts (pbjs does not create
  parent directories and failed with ENOENT)

Unblocks format, license-header, lint, and attw jobs which all depend
on @bufbuild/protobuf-benchmarks#generate succeeding.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>