docs: local read A/B re-run on an idle machine

The first run of the day could not get an idle tank: two orphaned h5py
SWMR reader processes from earlier interop tests (since stopped) kept a
core each busy. Re-run with the load below 2 at every round:
ObjectHeader::parse is +4.2% (real: the ranges do not overlap; about 2.5
ns per header, not visible in the facade listing, which is -1.7%); full
deflate reads +1.7% (1 thread) to +36% (16 threads); the -5.6% single-
thread contiguous hyperslab result from the loaded run is noise (-1.7%,
overlapping ranges) and is withdrawn from known-issues.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

CHANGELOG.md

@@ -477,19 +477,19 @@ Design: `docs/design/swmr.md`.
   `main` and it calls only format-crate code; its earlier +5–10% (323–355
   vs 353–363 ns) was run-to-run layout noise (at `2893b6c` it measured
   357–374 ns against `main`'s 355–371 in the same rounds).
-  **Rechecked 2026-09-27** (still provisional: two orphaned h5py test
-  processes kept tank's 1-minute load at 2.1–2.6, the load < 2 gate was
-  not met in 2 hours, load 2.3–3.3 during the runs; `8f59b2e` vs `main`
-  `7a8fae0` as separate binaries, pinned to one CPU, 6 alternating rounds,
-  median (range) over rounds; `BENCHMARKS.md`, "Local metadata and data
-  reads after range-read M2/M3"): facade listing 8.26 (8.08–8.45) → 8.20
-  (8.14–8.36) ms, −0.8%, so the listing regression is gone; symbol-table
-  nodes −0.4%, B-tree walk +0.5%; `ObjectHeader::parse` ×401 24.13
-  (23.97–25.56) → 25.11 (24.73–25.21) µs, **+4.1%, still open**
-  (`docs/known-issues.md`). Data reads (`concurrent_read --decode-threads
-  1`, 3 alternating rounds): full reads of deflate data 7–23% faster,
-  contiguous full reads and deflate hyperslabs within ±2%, one-thread
-  contiguous hyperslabs −5.6% (open, same entry).
+  **Rechecked 2026-09-27 on an idle tank** (load below 2 at every round;
+  `8f59b2e` vs `main` `7a8fae0` as separate binaries, pinned to one CPU, 3
+  alternating rounds, median (range); `BENCHMARKS.md`, "Local metadata and
+  data reads after range-read M2/M3"): facade listing 8.24 (8.04–8.28) →
+  8.10 (8.04–8.13) ms, **−1.7%**, so the listing regression is gone;
+  symbol-table nodes −0.5%, B-tree walk +1.1%; `ObjectHeader::parse` ×401
+  23.86 (23.79–23.96) → 24.86 (24.69–24.98) µs, **+4.2%, open**
+  (`docs/known-issues.md`; about 2.5 ns per header, not visible in the
+  listing). Data reads (`concurrent_read --decode-threads 1`): full reads
+  of deflate data +1.7% (1 thread) to +36% (16 threads), contiguous reads
+  and deflate hyperslabs within ±2%. (A run earlier that day under load
+  2.3–3.3 put single-thread contiguous hyperslabs at −5.6%; the idle rerun
+  does not reproduce it.)
 - Tests (2026-09-26, tank):
   - `clawhdf5-format/tests/storage_equivalence.rs` now also reads every
     dataset — whole, fill-aware, through a chunk cache (twice) and the
@@ -644,10 +644,9 @@ Design: `docs/design/swmr.md`.
   instead of being inlined into the generic parser. Provisional (tank load
   3–4; Criterion, separate binaries, 2 alternating rounds): 24.96–25.12 vs
   24.85–25.15 µs on `main`.
-  Rechecked 2026-09-27 with 6 alternating rounds (load 2.3–3.3, still
-  provisional): 24.73–25.21 µs against 23.97–24.29 µs (one outlier 25.56)
-  for `8f59b2e`, median +4.1%; a residual cost remains (see the M2 speed
-  note and `docs/known-issues.md`).
+  Rechecked 2026-09-27 on an idle tank (3 alternating rounds, load below
+  2): 24.69–24.98 µs against 23.79–23.96 µs for `8f59b2e`, median +4.2%; a
+  residual cost remains (see the M2 speed note and `docs/known-issues.md`).
 - Tests: `crates/clawhdf5-tools/tests/edit_coverage_interop.rs` (h5py
   `earliest`/`v110`/`latest` and clawhdf5-written files; structure
   comparisons with libhdf5 for version-2 B-trees, shrink on every index,