gates: verdict pipeline — gates + bench + ledger row, pristine-rebuilt baselines

Adds crates/verdict ('just verdict'): runs the core gates on the
candidate binary, then an interleaved A/B bench vs the baseline with
the CI-lower-bound rule, and writes the append-only ledger row with
per-side median CIs, workload statement, and env fingerprint. Gate
failure short-circuits to rejected-gate; --needs-human-review caps an
accepted verdict (never upgrades). CPU pinning moves into accept.toml
(pin_prefix, machine-specific, applied to both sides identically).

The smoke test caught a real trust gap: pointing --baseline-bin at the
workspace build (phase1-verdict-smoke-001) measured patched-vs-patched
and returned null, because gate runs rebuild the workspace binary from
the patched tree. Fix: --rebuild-baseline stashes the patch, builds
the baseline into isolated targets/<name>/baseline/, and restores the
patch — SPEC 3.1's pristine-checkout rule, now enforced by the
harness. Re-run (phase1-verdict-smoke-002) reproduces the Phase 0
result mechanically: +10.4% median, 95% CI [1.0932, 1.1137], accepted.
Both smoke rows are in the ledger, including the null one.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EcKPWQ3MQD4NdEHwar96u2

crates/bench-runner/src/config.rs

@@ -18,6 +18,11 @@ pub struct AcceptConfig {
     pub confidence: f64,
     /// PRNG seed for the bootstrap (reproducible calibration).
     pub bootstrap_seed: u64,
+    /// Machine-specific command prefix for CPU pinning (e.g.
+    /// "taskset -c 2"); empty disables pinning. Applied by verdict to
+    /// both sides identically.
+    #[serde(default)]
+    pub pin_prefix: String,
 }
 
 impl Default for AcceptConfig {
@@ -29,6 +34,7 @@ impl Default for AcceptConfig {
             bootstrap_iters: 10_000,
             confidence: 0.95,
             bootstrap_seed: 0x707E_17E5,
+            pin_prefix: String::new(),
         }
     }
 }