fix(ci): build the clang-tidy lanes without LTO so the gate can pass at all

[lusoris]

Summary

The required Tidy Changed and Tidy Ratchet gates have failed on every C/C++ pull request since #1290 merged, whatever the diff contained — the queue could not merge a single C change.

Cause, and it is mine: #1290 (ADR-1172) added b_lto_threads=4 to core/meson.build's default_options to cap link-time parallelism on the workstation. Meson renders that as GCC's -flto=4. The tidy lane configures with CC=gcc-14, so the flag lands in compile_commands.json — and clang-tidy parses those entries with clang, which rejects the argument outright:

clang: error: unsupported argument '4' to option '-flto='

Every translation unit therefore ended in 1 error generated, and the gate could never pass. Reproduced against a stock origin/master build: 1257 -flto=4 entries in compile_commands.json, and clang-tidy -p core/build core/src/picture.c → Error while processing.

Both tidy builds now configure with -Db_lto=false. They exist only to emit compile_commands.json and the generated headers, so LTO buys nothing there, and the SYCL tidy lane already configured that way for its own toolchain reason. After the change compile_commands.json holds zero -flto entries and the same file processes cleanly (459 suppressed, no error). Release builds keep the ADR-1172 cap unchanged.

Note for reviewers: the branch of #1312 carries its own copy of the T-TIDY-CHANGED-LTO-FLAG-2026-09-05 row, written while its agent was diagnosing the same failure. When #1312 rebases on this, drop its copy — the ADR-0165 duplicate-row gate added in #1322 will otherwise flag it.

Type

• fix — CI gate unblocked

Checklist

• Commits follow Conventional Commits.
• make format && make lint is green locally (pre-commit on every touched file; actionlint reports only the seven SC2317 notes that are already on master).
• Unit tests: n/a — a workflow configure flag; the reproducer below is the test.
• Docs — no docs needed: CI-internal build flag; the invariant is recorded in docs/rebase-notes.md and the workflow comment.
• SIMD/GPU, twins, new C sources, breaking change, ADR — n/a (ADR-1172 stands; only the tidy lane opts out).

Bug-status hygiene (ADR-0165)

• docs/state.md — T-TIDY-CHANGED-LTO-FLAG-2026-09-05 added to Recently closed.

Netflix golden-data gate (ADR-0024)

• I did not modify any assertAlmostEqual(...) score in the Netflix golden Python tests.

Deep-dive deliverables (ADR-0108)

• Research digest — no digest needed: one incompatible flag, reproduced in the state.md row and below.
• Decision matrix — no alternatives: only-one-way fix. The tidy build needs no LTO; stripping the flag from compile_commands.json afterwards, or reverting ADR-1172, would both be worse.
• AGENTS.md invariant note — no rebase-sensitive invariants: the coupling is a rebase note (any compile-commands-only build configures -Db_lto=false).
• Reproducer / smoke-test command — below.
• CHANGELOG fragment — changelog.d/fixed/tidy-lane-lto-flag.md.
• Rebase note — docs/rebase-notes.md entry.

Reproducer

printf 'int main(void){return 0;}\n' > /tmp/t.c
clang -flto=4 -c /tmp/t.c -o /dev/null     # error: unsupported argument '4' to option '-flto='
gcc   -flto=4 -c /tmp/t.c -o /dev/null     # accepted

# before: a stock master build
meson setup /tmp/b core -Denable_cuda=false -Denable_sycl=false && meson compile -C /tmp/b
grep -c 'flto=4' /tmp/b/compile_commands.json          # 1257
clang-tidy -p /tmp/b core/src/picture.c                # "Error while processing"

# after: what the lane now does
meson setup /tmp/b2 core -Denable_cuda=false -Denable_sycl=false -Db_lto=false && meson compile -C /tmp/b2
grep -c 'flto' /tmp/b2/compile_commands.json           # 0
clang-tidy -p /tmp/b2 core/src/picture.c               # processes cleanly

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