bench: add CLP vetting harness and codec-8 corpus gate

tools/clp-vet: the Phase-0 measurement harness behind the codec-8
GO decision (CLP_PLAN.md) plus codec8_bench, which drives the real
encode_block/decode_block on rich entries built from the corpora and
regression-gates codec 8 vs codec 7 (bytes/entry, encode/decode MB/s,
per-block fallback counts, exactness asserts). Corpora are git-ignored
(Loghub sets re-fetchable, journal slices host-local);
results_phase0.txt is the tracked evidence. Also refreshes the stale
0.4.0 workspace pins in tools/bench/Cargo.lock.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: awksedgreep

tools/bench/Cargo.lock

@@ -0,0 +1,5 @@
+# Corpora are not committed: Loghub sets are re-fetchable (see CLP_PLAN.md
+# Phase 0), journal slices are host-local captures. results_phase0.txt IS
+# tracked — it is the decision-log evidence.
+corpora/
+target/