masking-parity: the same wrong fallback claim, at its source

CodeRabbit on #314. `scripts/masking-parity.sh:26-30` carried the identical
claim this PR already corrected in CLAUDE.md: that a set RUSTFLAGS makes the
arm "measure v3 while claiming v4".

Sharper than a duplicate: CLAUDE.md's correction CITES this script as the
place the trap is documented, so the doc was fixed while the source it points
at still taught the wrong number. An exhaustive grep confirms these were the
only two instances outside the blackboard's historical record.

The correction, with this session's own measurement: RUSTFLAGS replaces EVERY
config rustflags entry, including the DEFAULT config's own `-Ctarget-cpu`, so
no target-cpu reaches rustc and the build lands on the generic `x86-64`
baseline (SSE2) — BELOW v3, and the tier `simd_avx2.rs`'s intrinsics SIGILL
on. Measured: `RUSTFLAGS="-D warnings"` emitted ZERO `-Ctarget-cpu` flags
against 65 with the env unset.

Also states what the arm now IS, since the default flip: it names no tier and
builds whatever config wins, by default `target-cpu=native`. Read the
program's own `avx512f=` header for the tier, or pin config-v3 for AVX2.

Verified the script still runs both ways:
  bash scripts/masking-parity.sh native                    PASS, avx512f=true
  CARGO_ARGS='--config .cargo/config-v3.toml' ... native    PASS, avx512f=false

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DCfrD5y19cvFc4AoyydXYv

Author: claude

scripts/masking-parity.sh

@@ -26,8 +26,21 @@ case "$ARM" in
     # `env -u RUSTFLAGS`: a workflow-global RUSTFLAGS (CI sets "-D warnings")
     # REPLACES every cargo-config rustflags entry, so a `--config
     # .cargo/config-v4.toml` passed through CARGO_ARGS would silently lose its
-    # `-Ctarget-cpu=x86-64-v4` and this arm would measure v3 while claiming
-    # v4 — the exact trap the tier4 CI job hit. Clearing it lets the config win.
+    # `-Ctarget-cpu=x86-64-v4` — the exact trap the tier4 CI job hit. Clearing
+    # it lets the config win.
+    #
+    # What you get INSTEAD is not v3 (corrected 2026-09-16, coderabbit on #314;
+    # this comment said "would measure v3"). RUSTFLAGS replaces EVERY entry,
+    # including the DEFAULT `.cargo/config.toml`'s own `-Ctarget-cpu`, so no
+    # target-cpu reaches rustc at all and the build lands on rustc's generic
+    # `x86-64` baseline — SSE2, BELOW v3, and the tier `simd_avx2.rs`'s
+    # intrinsics SIGILL on. Measured on one unit: `RUSTFLAGS="-D warnings"`
+    # emitted ZERO `-Ctarget-cpu` flags against 65 with the env unset.
+    #
+    # This arm NAMES NO TIER by design: it builds with whatever config wins,
+    # which by default is `target-cpu=native` (the host). Read the program's
+    # own `avx512f=` header line for the tier; pin `.cargo/config-v3.toml`
+    # through CARGO_ARGS when you specifically want AVX2.
     env -u RUSTFLAGS cargo ${CARGO_ARGS:-} build --release --manifest-path "$MANIFEST" --bin simd-masking-parity
     "$TD/release/simd-masking-parity"
     ;;