phase0: exit criteria met — tokei +10.4% median [8.5%, 12.0%] verified

Attempt 4 (phase0-tokei-002, class 3, ACCEPTED — first auto-accept):
every file was read through encoding_rs_io's DecodeReaderBytes via
generic read_to_end, which zero-fills every grown buffer chunk (~221MB
memset/run, 8.5% of Ir) and grows without a size hint. Patch: fs::read
with a BOM-gated fallback to the identical decode path (a UTF-8-BOM
file transcodes invalid sequences, so the fallback is required for
equivalence). Gates: golden byte-identical; differential fuzz old-vs-
new binaries 4,332 iterations (BOM grid, invalid UTF-8, binary junk,
randomized) zero divergence; 250 upstream tests green; ASan+LSan clean
with ASan-run output hashing to golden. Bench: +9.9% [8.0%, 12.7%].

Attempt 5 (phase0-tokei-003, ship tree, ACCEPTED): stacks fat LTO+cgu=1
(real +1.6% standalone but under threshold, phase0-tokei-001) on the
accepted patch; benched as a whole vs pristine: +10.4% [8.5%, 12.0%].
Median clears the Phase 0 exit bar; CI lower bound 8.5%.

Case study written (results/phase0/case-study-tokei.md), ledger at 5
attempts, CLAUDE.md phase line moved to Phase 1, next hypotheses
logged (tokei class-5 byte-skip table; comrak class-3 arena).

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

results/phase0/case-study-tokei.md

@@ -0,0 +1,79 @@
+# Phase 0 case study — tokei (source-code statistics)
+
+**Result: +10.4% median wall-time speedup (95% CI [+8.5%, +12.0%]) vs
+pristine, byte-identical output, 4,332-iteration differential fuzz
+clean, sanitizers clean, auto-accepted. Phase 0 exit criterion
+(≥1 verified win ≥10%) met by the median; the CI lower bound is 8.5%.**
+
+## Target & workload
+
+- tokei v14.0.0, commit `fa44e519`, MIT OR Apache-2.0.
+- Workload: 8 replicas of two pinned trees (progit @ `61833a52`,
+  comrak src @ `45c1995f`), 221 MB / 4,360 files, `--sort code`,
+  `RAYON_NUM_THREADS=1` (measurement first; multithreaded scaling is a
+  class-7 concern). Corpus pinned: `corpora/tokei/MANIFEST.sha256`.
+- Environment: as case-study-comrak.md (pinned core, interleaved ABAB,
+  30+3 runs/side, bootstrap 95% CI; A/A null [0.9891, 1.0118]).
+
+## The find (profile → hypothesis)
+
+callgrind put **8.5% of instructions in `__memset_avx2`** with heavy
+malloc/memcpy alongside — odd for a tool that just reads and scans
+files. Reading `LanguageType::parse` explained it: every file is read
+through `encoding_rs_io::DecodeReaderBytes` via generic `read_to_end`.
+That reader has no `read_buf` specialization, so std **zero-fills every
+grown buffer chunk before the read lands** (~221 MB of memset per run),
+and the Vec grows without a file-size hint (realloc + memcpy churn).
+The decoding wrapper only *does* anything when the file starts with a
+BOM — for BOM-less input it is byte-for-byte passthrough.
+
+## The patch (results/phase0/tokei-readpath.patch, ~20 lines)
+
+`fs::read` (pre-sized from metadata, `read_buf`-specialized, no
+zeroing) for every file; files starting with a UTF-8/UTF-16 BOM fall
+back to the *original* `DecodeReaderBytes` machinery over the same
+bytes. Equivalence argument: encoding_rs_io BOM-sniffs the stream head;
+identical bytes in → identical bytes out on both paths. The subtle case
+that forbids skipping the fallback: a UTF-8-BOM file is actually
+transcoded (invalid sequences → U+FFFD), unlike BOM-less input.
+
+## Gates
+
+- Golden replay: stdout sha256 identical to pinned golden.
+- **Differential fuzz, old binary vs new binary**: crafted grid (7 BOM
+  prefixes × 12 bodies × 2 extensions — including invalid UTF-8,
+  UTF-16LE/BE, truncated BOMs, binary junk, 70 KB single lines,
+  unterminated strings/comments) plus randomized cases; **4,332
+  iterations / >90 s, zero divergence** in either default or JSON
+  output.
+- Upstream test suite: 250 tests green on the patched tree.
+- ASan + LSan (nightly) over the full 221 MB corpus: clean; the
+  ASan-instrumented binary's output hashes to golden.
+
+## Bench (ledger rows phase0-tokei-002 / -003)
+
+| candidate | median speedup | 95% CI | verdict |
+|---|---|---|---|
+| read-path patch alone | 1.0990 | [1.0797, 1.1271] | accepted |
+| + fat LTO, cgu=1 (ship tree) | **1.1036** | **[1.0852, 1.1201]** | accepted |
+
+Fat LTO alone had measured a real but sub-threshold +1.6% [1.0026,
+1.0262] (phase0-tokei-001, rejected standalone). The ship tree stacks
+it on the accepted patch and is benched as a whole against the pristine
+baseline — the bench measures exactly what ships.
+
+## ROI (illustrative, config/pricing.toml rates)
+
+Hypothetical 100-core 24/7 fleet on this workload shape at $0.04/core-hr:
+cores returned median **9.4**, 95% CI [7.9, 10.7]; dollars/yr median
+**$3,290**, 95% CI [$2,750, $3,760]. Caveats: single workload shape
+(cold-ish batch scan of a large mixed tree, warm page cache, single
+thread); Ir-profile prioritization; container hardware, no governor
+control; results include the codegen (LTO) layer which is
+toolchain-version-sensitive (rustc 1.94.1).
+
+## Upstreamability
+
+The read-path change is a candidate for an upstream tokei PR
+(BOM-gated fast path, no behavior change, 250 tests + fuzz evidence).
+Not filed from this session; listed as follow-up.