fix: docs: document router-cache benchmark fixtures

benchmarks/corpus/router-cache/README.md

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+    # Router-cache benchmark fixtures
+
+    Checked-in corpus used by router-cache benchmarks so CI and local runs share the same
+    inputs (manifest, campaign, schedule, synthetic PowerPlan / observation samples).
+
+    ## Layout
+
+    - `campaign.json`
+- `cost-model.json`
+- `legacy-offline-campaign.json`
+- `legacy-offline-manifest.json`
+- `manifest.json`
+- `minimal-live-pilot.json`
+- `oracles/README.md`
+- `oracles/ripgrep-crlf-trace.json`
+- `oracles/ripgrep-glob-case-insensitive-locate.json`
+- `oracles/ripgrep-no-ignore-vcs-locate.json`
+- `oracles/ripgrep-null-data-trace.json`
+- `power-plan.json`
+- `schedule.json`
+- `synthetic-calibration-observations.jsonl`
+- `synthetic-final-observations.jsonl`
+- `synthetic-sealed/docs/ripgrep-crlf-trace-calibration.json`
+- `synthetic-sealed/docs/ripgrep-glob-case-insensitive-locate-calibration.json`
+- `synthetic-sealed/docs/ripgrep-no-ignore-vcs-locate-holdout.json`
+- `synthetic-sealed/docs/ripgrep-null-data-trace-holdout.json`
+- `synthetic-sealed/index.json`
+
+    ## How these are used
+
+    - Treat this tree as **read-only fixtures**: do not commit regenerated large artifacts
+      or machine-specific paths.
+    - Benchmarks should resolve paths relative to the repository root (or the package
+      `CARGO_MANIFEST_DIR`) so clones remain portable.
+    - When adding a fixture, keep payloads small, deterministic, and document purpose
+      plus which bench target consumes them.
+
+    ## Regenerating (if applicable)
+
+    Prefer committed golden files over ad-hoc generation. If a generator exists under
+    `benchmarks/`, document the exact command here and keep outputs stable across platforms.