phase5: kernel lane (SPEC §13) — FP-tolerance equivalence gate + reference-diff testing

The parser/service targets only ever needed byte-identical golden replay.
The kernel lane is the case that breaks it: a matmul optimization
(transpose-B for cache locality + eight-accumulator ILP) that REORDERS the
floating-point reduction, so its low bits differ by construction —
byte-identical is the wrong gate.

Adds:
  - EquivalencePolicy::compare + Divergence (diff-test): byte-identical is
    exact; fp-tolerance tokenizes and compares numeric tokens within
    abs + rel*|baseline|, with explicit NaN handling. Non-numeric tokens and
    token-count mismatches still fail exactly.
  - fp-compare bin: applies a target's equivalence.toml to two output files.
  - targets/matmul/kernel.c: matmul_ref (single-accumulator oracle) vs
    matmul_opt (reordered, faster), with emit/check/run/bench modes.
  - targets/matmul/equivalence.toml: fp-tolerance abs 1e-4 + rel 1e-3.
  - scripts/kernel-lane-demo.sh + results/phase5/kernel-lane.txt: byte-identical
    FAILs, fp-tolerance is EQUIVALENT, a +0.5 perturbation is still REJECTED,
    speedup 3.23× measured with the same interleaved A/B + bootstrap CI.

Ledger row phase5-matmul-opt = needs-human-review: using the FP-tolerance
tier at all is a human-review signal (SPEC §8), same posture as mimalloc.
The trust machinery is unchanged; only the timer and the gate differ, which
is exactly the shape a future GPU kernel would slot into.

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

scripts/kernel-lane-demo.sh

@@ -0,0 +1,60 @@
+#!/bin/sh
+# Kernel-lane demonstration (SPEC §13 research fork). Shows the one thing
+# the parser/service targets never needed: an optimization that reorders
+# floating-point accumulation, so byte-identical golden replay is the WRONG
+# gate and a floating-point TOLERANCE gate is required — while a genuine
+# wrong result is still caught. Measured with the same interleaved A/B +
+# bootstrap CI as every other target.
+#
+#   sh scripts/kernel-lane-demo.sh [n]
+set -eu
+ROOT=$(cd "$(dirname "$0")/.." && pwd)
+N=${1:-512}
+OUT="$ROOT/results/phase5"
+mkdir -p "$OUT" /tmp/kl
+
+cd "$ROOT"
+clang -O2 -o /tmp/kl/kernel targets/matmul/kernel.c -lm
+cargo build -q -p diff-test -p bench-runner
+
+{
+echo "# Kernel-lane demonstration — matmul (n=$N, f32)"
+echo
+echo "## 1. Equivalence: byte-identical is the wrong gate"
+/tmp/kl/kernel "$N" emit ref > /tmp/kl/ref.txt
+/tmp/kl/kernel "$N" emit opt > /tmp/kl/opt.txt
+if diff -q /tmp/kl/ref.txt /tmp/kl/opt.txt >/dev/null; then
+    echo "byte-identical: MATCH (unexpected — no reorder)"
+else
+    d=$(diff /tmp/kl/ref.txt /tmp/kl/opt.txt | grep -c '^<' || true)
+    echo "byte-identical: FAIL — $d of $((N*N)) result values differ (accumulation reordered)"
+fi
+echo
+echo "diff-test FP-tolerance policy (targets/matmul/equivalence.toml):"
+"$ROOT/target/debug/fp-compare" targets/matmul /tmp/kl/ref.txt /tmp/kl/opt.txt | sed 's/^/  /'
+echo
+echo "in-process reference differential test:"
+/tmp/kl/kernel "$N" check 1e-4 1e-3 | sed 's/^/  /'
+echo
+echo "control — a genuine wrong result IS caught by the tolerance:"
+awk 'NR==1{$1=$1+0.5} {print}' /tmp/kl/opt.txt > /tmp/kl/wrong.txt
+if "$ROOT/target/debug/fp-compare" targets/matmul /tmp/kl/ref.txt /tmp/kl/wrong.txt >/dev/null 2>&1; then
+    echo "  !! tolerance accepted a wrong result (bug)"
+else
+    echo "  tolerance correctly REJECTS a +0.5 perturbation of one element"
+fi
+echo
+echo "## 2. Speedup: interleaved A/B, bootstrap 95% CI"
+cargo run -q -p bench-runner -- --config config/accept.toml compare \
+    --baseline "taskset -c 2 /tmp/kl/kernel $N run ref" \
+    --candidate "taskset -c 2 /tmp/kl/kernel $N run opt" 2>/dev/null \
+    | grep -E "speedup|verdict" | sed 's/^/  /'
+echo
+echo "## What it demonstrates"
+echo "A real matmul speedup (transpose for cache locality + eight-accumulator"
+echo "ILP) that reorders the reduction. Byte-identical rejects it; the"
+echo "FP-tolerance gate accepts it (abs 1e-4 + rel 1e-3) while still catching"
+echo "a wrong result; the bench measures it with the same CI machinery as"
+echo "every other target. The GPU extension (Triton/CUDA vs a reference"
+echo "kernel) is the same shape — only the timer and the hardware change."
+} | tee "$OUT/kernel-lane.txt"