gates: close the audit findings — enforce the gates the docs claimed

An adversarial audit found the enforcement story ahead of the code: gates
that were documented as hard gates were advisory, manual, or absent, and
several human-facing claims outran the machine record. This makes the
claims true.

High findings

- Differential fuzz was documentation, not enforcement: verdict hardcoded
  fuzz_iters=0 and diff-test hardcoded the gate to Skipped, while the case
  studies claimed 10k-iteration fuzz on the same wins. Fuzz is now a real
  pipeline gate (strict FUZZ-RESULT contract, executed count recorded), and
  it is inherently baseline-vs-candidate, so it runs on the accept path and
  reports Skipped in the standalone `just gates` flow. New hard rule: a
  machine `accepted` verdict is impossible unless the fuzz gate actually
  passed — otherwise it caps at needs-human-review.
- The agent's system prompt claimed FP/concurrency/unsafe patches were
  "routed to human review automatically". No such mechanism existed. Added
  a conservative lexical risk classifier (crates/verdict/src/risk.rs) that
  forces needs-human-review on any would-be accept carrying those signals;
  fp-tolerance equivalence always signals.
- The local verdict path ran agent-influenced code unsandboxed. It now runs
  under scripts/no-net.sh (network namespace) and fails closed. Upstream
  test suites are hash-verified against corpora/<t>/TESTSUITE.sha256 when
  present (scripts/pin-testsuite.sh generates them).

Also fixed, found while fixing the above

- The pipeline script's `set -e` was inert (brace group as the left operand
  of `&&`), so a failing candidate build did not abort the run and verdict
  could bench a stale binary. Now a scoped subshell whose status is branched
  on at top level.
- A wrapper that exits before exec'ing (no-net.sh refusing, bad wrapper path)
  stranded RUNNING.lock and left read_verdict polling forever. Failure
  accounting now lives outside the wrapper.
- harnessd's patch allowlist ignored git rename/copy headers and --no-prefix
  diffs; the rename-escape case is now a committed regression test.
- The workspace did not compile on Windows (ungated Unix APIs). cfg-gated;
  71 portable tests now run on both platforms.
- pin-check regenerated the corpus manifest instead of verifying it; it is
  now verify-only, with a deliberate `just pin-corpus` to re-pin.
- Max RSS is captured per run on Unix; PMU counters are opt-in diagnostics
  (compare --perf-stat), never part of the accept decision.

Docs now match the code, including what is NOT yet enforced: no target ships
a TESTSUITE.sha256 pin, no target declares [build].tsan, and matmul is not
wired to the fp-tolerance gate — each is stated as mechanized-but-inert
rather than done. Corrected the stale "33 ledger rows" (34), the playbook's
class-3 cJSON win (rejected), adversarial-review.md's sanitizer-separation
claim (five accepted rows carry sanitizers_clean=false), the unsupported
"40,000 requests" figure, and the blanket "historical rows record
fuzz_iters=0" (8 of 10; phase0-tokei-002/-003 record 4,332).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/pin-testsuite.sh

@@ -0,0 +1,63 @@
+#!/bin/sh
+# Pin an upstream test suite: hash every file under the given repo-root-
+# relative paths (typically targets/<t>/workspace/tests/...) into
+# corpora/<target>/TESTSUITE.sha256, sha256sum two-space format. diff-test
+# verifies this pin before running upstream tests; targets/fetch.sh
+# verifies it after every fetch. Run from repo root:
+#   scripts/pin-testsuite.sh <target> <path>...
+set -eu
+if [ $# -lt 2 ]; then
+    echo "usage: scripts/pin-testsuite.sh <target> <path>..." >&2
+    exit 2
+fi
+target="$1"
+shift
+out="corpora/$target/TESTSUITE.sha256"
+if [ ! -d "corpora/$target" ]; then
+    echo "pin-testsuite: corpora/$target does not exist (run from repo root?)" >&2
+    exit 1
+fi
+for p in "$@"; do
+    if [ ! -e "$p" ]; then
+        echo "pin-testsuite: $p not found (paths are repo-root-relative)" >&2
+        exit 1
+    fi
+done
+
+# Build the pin out-of-place and install it only on success: a run that fails
+# anywhere below must not truncate, replace, or half-write an existing pin —
+# that pin is what diff-test refuses to run against on mismatch.
+list="$out.files.$$"
+tmp="$out.new.$$"
+trap 'rm -f "$list" "$tmp"' EXIT INT TERM
+
+# LC_ALL=C: byte-order sort, so the same tree pins identically on every
+# machine. -print0/-z: filenames with spaces or newlines survive intact.
+find "$@" -type f -print0 | LC_ALL=C sort -z > "$list"
+count=$(tr -dc '\0' < "$list" | wc -c | tr -d ' ')
+if [ "$count" -eq 0 ]; then
+    # Was: `find ... | xargs -0 sha256sum > "$out"` — with no matches, xargs
+    # still ran sha256sum with zero file arguments, so it hashed STDIN (the
+    # empty pipe) and wrote a bogus "<sha-of-empty>  -" entry, then reported
+    # SUCCESS. That is a pin of nothing that later "verifies" by reading
+    # stdin. Refuse instead, and leave any existing pin untouched.
+    echo "pin-testsuite: no regular files under: $* — refusing to write $out" >&2
+    exit 1
+fi
+
+# --text is GNU's default on Linux (a no-op there); it is spelled out because
+# sha256sum defaults to *binary* mode on some platforms, emitting "<hash> *path"
+# — a one-space-plus-asterisk form that diff-test's pin verifier (which splits
+# on the two-space separator) does not parse. Paths stay repo-root-relative:
+# find was handed repo-root-relative paths and we run from the repo root.
+xargs -0 sha256sum --text -- < "$list" > "$tmp"
+
+lines=$(wc -l < "$tmp" | tr -d ' ')
+if [ "$lines" -ne "$count" ]; then
+    echo "pin-testsuite: hashed $lines of $count files — refusing to write a partial pin" >&2
+    exit 1
+fi
+mv "$tmp" "$out"
+echo "wrote $out ($count files)"
+echo "pinning a test suite is a deliberate human action (corpora/README.md):"
+echo "commit the new pin with justification."