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/diff-test/src/target.rs

@@ -9,10 +9,19 @@ use std::path::{Path, PathBuf};
 pub struct TargetSpec {
     pub source: Source,
     pub build: Build,
+    pub bench: Bench,
     pub gates: Gates,
     pub corpus: Corpus,
 }
 
+#[derive(Debug, Deserialize)]
+pub struct Bench {
+    /// Bench command; `{binary}` substituted per side. Run from repo root.
+    pub command: String,
+    /// Human-readable workload statement, printed with every number.
+    pub workload: String,
+}
+
 #[derive(Debug, Deserialize)]
 pub struct Source {
     pub repo: String,