Add measure-only prober leaf director

[TheGreatAxios]

Summary

• Adds a closed leaf director prober: measure-only lane reporting TTFT, per-turn latency, tool-only streak distribution, and salvage/nudge counts per family/model.
• Consumes the existing harness only: bun run eval:capability (scripts/eval-capability.ts over evals/capability) and the PerfTrace harness under src/perf (rollup, assert-spans, attribution-report). No second harness.
• Findings feed model-family-policy as follow-up tickets; the lane forbids shipping product code and tuning prompts or policy (never silent retunes). Leaf: maySpawn false.
• Registry + prompt-contract tests; no Skywalker changes.

Verification

• bun run typecheck, bun run build, and bun run test pass (via bun run check: lint + typecheck + build + guarded suite, 7146 pass / 0 fail)
• Related suites: prober package contract, director registry, prompt-size budgets — 39 pass / 0 fail

Related to CL-7656

[linear-code]

CL-7656

[TheGreatAxios]

Audit (Prober director): Approve with nits.

Findings:

• Budgets + routing present; identity follows convention (You are ProberDirector (Prober)).
• Docs: none on this PR — after Add a Gauntlet director that mutation-checks tests can actually fail #977 lands as the template, consider a small docs follow-up matching its shape if the team wants parity.
• Collision: same 16->17 bump as Add a Gauntlet director that mutation-checks tests can actually fail #977/Add Migrator director for reversible data migrations #963/Add warden trust-review director for permission and plugin paths #966 in types.ts, registry.test.ts, registry.ts, skywalker allowlist, prompt-sizes.test.ts.
• Sequencing: land Add a Gauntlet director that mutation-checks tests can actually fail #977 first, then rebase this PR to 18/19/20 during merge phase. Do NOT stack onto sibling branches.
• No code fix required on this PR — comment only.