feat(perf,benchmark-infra): criterion bench harness + advisory CI workflows (SDD-120)

Implementation of SDD-120 §1-§8:

Bench harness (crates/chanora_audio/benches/):
- common.rs: deterministic synthetic audio (440 Hz sine, no RNG).
- realtime_capture.rs: bench_capture_alloc_count (dhat) +
  bench_capture_callback_wall_clock (criterion).
- opus_codec.rs: bench_opus_encode_latency + bench_opus_decode_latency
  (direct audiopus, not AudioHandler — SDD-120 §3 item 4).
- resampler.rs: bench_resampler_throughput across 44.1->48 /
  16->48 / 48->48 passthrough.

CI tooling (crates/chanora_audio/examples/):
- emit_baseline.rs: aggregates criterion estimates.json outputs
  into the SRS-217 baseline schema.
- compare_baseline.rs: applies SRS-219 tolerance, renders markdown
  table with 🟢/🟡/🔴 markers + yellow simpler-form realization per
  SDD-120 §8.

  Deviation from SDD-120 §2 / §5 / §7 placement: these tools live
  under examples/, not benches/ or src/bin/. Rationale: they must
  consume serde_json (a dev-only dep — production builds must not
  pull it). Cargo only resolves dev-dependencies for [[test]],
  [[bench]], and [[example]] targets; [[bin]] targets under
  src/bin/ see only regular [dependencies]. examples/ keeps the
  binaries out of the production dep tree while still giving them
  cargo run --example invocation. An SDD-120 amendment should
  reflect this.

Workflows (.github/workflows/):
- bench-advisory.yml: PR + push triggers; runs benches; posts a
  sticky PR comment via actions/github-script@v7; job status is
  always success (SRS-218 clause 4 — non-blocking).
- bench-baseline-update.yml: workflow_dispatch only; runs benches;
  opens PR via peter-evans/create-pull-request@v6 (sole writer of
  the SAD-089 baseline JSON).

Cargo.toml additions ([dev-dependencies] only — verified excluded
from --release builds): criterion 0.5, dhat 0.3, serde_json 1.

Source-code seam: minimal pub-but-#[doc(hidden)] bench_seam module
in chanora_audio (engine.rs + lib.rs re-export) so the criterion
bench harness can construct a CaptureState and drive
CaptureState::ingest without re-implementing the engine (SDD-120
§3). Non-iOS targets only — CaptureState itself is iOS-gated.

Initial baseline seed: crates/chanora_audio/benches/baselines/
x86_64-unknown-linux-gnu.json = {}. compare_baseline handles the
missing-baseline case gracefully and emits a 'no red markers'
report; the first manual dispatch of bench-baseline-update.yml
after merge establishes the real values.

Out of scope per SDD-120 §10: production telemetry export,
build-failing hard CI gate, multi-host benchmarking, IDE
integration, Dart-side bridge round-trip bench.

Verification:
- cargo check --workspace --all-targets: PASS.
- cargo bench --bench realtime_capture --no-run: PASS.
- cargo bench --bench opus_codec --no-run: PASS.
- cargo bench --bench resampler --no-run: PASS.
- cargo build --example emit_baseline --example compare_baseline
  -p chanora_audio: PASS.
- cargo test --workspace: 106 passed, 0 failed, 3 ignored — no
  regression from prior count.

.github/workflows/bench-baseline-update.yml

@@ -0,0 +1,53 @@
+name: bench-baseline-update
+
+# SDD-120 §7 / SAD-089 — manual-dispatch workflow that opens a PR
+# updating the committed baseline JSON. This is the SOLE writer of
+# `crates/chanora_audio/benches/baselines/<host>.json`. Triggered
+# only on operator demand via workflow_dispatch.
+
+on:
+  workflow_dispatch:
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  bench-baseline-update:
+    name: bench-baseline-update
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: System deps (cpal / Opus)
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y \
+            libasound2-dev libpulse-dev pkg-config \
+            libopus-dev
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@v2
+      - name: Run benchmarks
+        run: |
+          cargo bench -p chanora_audio \
+            --bench realtime_capture \
+            --bench opus_codec \
+            --bench resampler
+      - name: Emit current baseline JSON
+        run: cargo run --example emit_baseline -p chanora_audio
+      - name: Copy current to baseline path
+        run: cp current.json crates/chanora_audio/benches/baselines/x86_64-unknown-linux-gnu.json
+      - name: Create pull request
+        uses: peter-evans/create-pull-request@v6
+        with:
+          branch: bench/baseline-update-${{ github.run_id }}
+          base: product/scaffold-v0
+          title: "chore(bench): update realtime audio baselines"
+          body: |
+            Automated baseline update produced by manual dispatch of
+            `bench-baseline-update.yml`.
+
+            Run: ${{ github.run_id }}
+            Triggered by: ${{ github.actor }}
+          commit-message: "chore(bench): update realtime audio baselines"
+          add-paths: |
+            crates/chanora_audio/benches/baselines/x86_64-unknown-linux-gnu.json