ci: move gcc forward with the rest of the toolchain, and record it (ADR-1230)

[lusoris]

Summary

The clang-tidy lanes install three toolchain components and treated them
inconsistently:

Tool	ubuntu-24.04 ships	What CI did	Rationale in the workflow
clang	18 — can't parse the tree's C++26	installs clang-22 from apt.llvm.org	yes, commented
meson	1.3.2 — predates c23 in c_std	installs from PyPI	yes, commented
gcc	14	used gcc-14	none

Two of three are deliberately pulled past the distro because the tree targets
C23/C++26 (ADR-0692). gcc was left at whatever the runner image shipped. No ADR
pins it — the only gcc-14 mention in docs/adr/ is an incidental line in a
VVenC changelog summary.

This stopped being cosmetic on #1392

The ratchet reported test_pooling_percentile.c: warnings 0 -> 1 (+1) while
Tidy Changed called the same file clean — same runner, same commit, and
that file is not in its exclusion list. It could not be reproduced on gcc-15 or
gcc-16; on both, the only warning was a glibc system-header diagnostic the
ratchet correctly discards.

The cause is structural: clang-tidy parses each TU against the system headers
the C compiler provides, so a ratchet count depends on gcc's version as much
as on clang-tidy's — and the baseline recorded only clang_tidy_version.
Pinning gcc a major behind every developer's machine while recording nothing
about it makes such a disagreement unexplainable.

The report artifact made it worse: parse_diagnostics() already produces every
diagnostic and the script threw all but the count away, so "which warning?"
had no answer anywhere in CI's output.

Three changes

1. gcc-15/g++-15 from ppa:ubuntu-toolchain-r/test on the clang-tidy
   lanes — the same treatment clang and meson already get.
2. The ratchet records cc_version, read from the build directory's
   meson-info/intro-compilers.json so it's the compiler that actually
   produced compile_commands.json, and annotates a mismatch exactly as a
   clang-tidy mismatch already did.
3. The --report artifact keeps every diagnostic as
   path:line:col: [check]. The baseline stays counts-only so it remains
   reviewable and doesn't churn on line-number shifts.

Reproducer / smoke-test command

meson setup core/build core -Denable_cuda=false -Denable_sycl=false -Db_lto=false
ninja -C core/build
python3 scripts/ci/tidy-ratchet.py --lane cpu --build-dir core/build \
  --only core/src/output.cpp --report /tmp/r.json
python3 -c "import json;d=json.load(open('/tmp/r.json'));print(d['cc_version']);print(len(d['diagnostics']),'diagnostics');print(d['diagnostics'][:3])"

Locally that prints cc (GCC) 16.2.1 20260810 and 29 diagnostics naming check
and location — the information #1392 needed and could not get.

⚠️ Requires a CI-side baseline regeneration

Counts under gcc-15's headers will not equal gcc-14's, so
scripts/ci/tidy-baseline-cpu.json is stale the moment this merges. It must be
rewritten from a CI run, never from a developer machine — a local --write
resolves enable_dnn=auto differently and would rebaseline unrelated files
against the wrong build. Expect this PR's own Tidy Ratchet to fail until that
happens; that failure is the point.

cc_version is optional on read, so the existing baseline loads unchanged and
simply doesn't trigger the mismatch warning until regenerated.

Deliberately out of scope

libvmaf-build-matrix.yml still pins gcc-14 across its legs. Bumping the
product build's compiler surfaces its own crop of new warnings across ten-plus
matrix legs and belongs in a PR whose CI failures are about exactly that.

Deep-dive deliverables (ADR-0108)

• Research digest — no digest needed: the investigation is ADR-1230 ## Context, and its evidence is feat(core): add percentile pooling methods (median, perc5, perc10, perc20) (ADR-1181) #1392's own CI output.
• Decision matrix — ADR-1230 ## Alternatives considered (five options, including the ubuntu-26.04 runner and baselining diagnostics).
• AGENTS.md invariant note — no rebase-sensitive invariants.
• Reproducer / smoke-test command — above.
• Changelog fragment — changelog.d/changed/1230-modern-gcc-toolchain.md, CHANGELOG.md regenerated.
• Rebase note — no rebase impact: fork-local CI configuration and tooling, no upstream counterpart.

State (rule 13)

no state delta: CI toolchain configuration, no product bug opened or closed.

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