S24.13: switch IWYU lanes to advisory

[DavidCozens]

Parent epic: #254 (E24: Code Hygiene)

Switch the two IWYU CI lanes (analyze-iwyu, analyze-iwyu-freertos-plustcp)
to advisory — they still run on every PR and upload their artifacts, but
findings no longer fail the build.

Motivation

A trial swap from IWYU to clang-tidy --checks=misc-include-cleaner
(the originally-scoped story) revealed that misc-include-cleaner's
built-in stdlib mapping has substantial gaps on POSIX extension symbols
(CLOCK_REALTIME, SOL_SOCKET, SO_KEEPALIVE, POSIX timeval, etc.),
producing roughly 75 % false positives on the Platform/Posix/ tree
(10 false positives vs 3 real findings in a 13-finding sample). The
check's only configuration knobs are IgnoreHeaders (per-header global
silencing) and DeduplicateFindings; no IWYU-style mapping file
support. So switching tools didn't actually solve the problem — it
exchanged one accuracy hit for another.

The simpler, higher-throughput move is to keep IWYU but stop it
blocking PRs:

• Per-PR friction (IWYU's two-pass "should add / should remove" output)
  costs a measurable chunk of every CI cycle.
• The actual signal — when IWYU is right — is best dealt with in
  batches around release prep, not in dribs and drabs in feature PRs.
• cppcheck stays required: its false-positive rate is low and S24.12
  just audited its suppressions.

The misc-include-cleaner / clang-include-cleaner standalone /
alternate-tool investigation moves to a separate candidate idea under
E24, surfaced if we revisit the swap.

Architecture boundary

CI workflow only. No production-code or test-code changes. Two
continue-on-error: true additions on the IWYU Run steps in
.github/workflows/ci.yml, plus a docs/local-checks.md refresh to
reflect the new tier shape.

Acceptance criteria

• analyze-iwyu and analyze-iwyu-freertos-plustcp lanes still
  run on every PR.
• Both lanes still upload their report artifacts
  (iwyu-report, iwyu-report-freertos-plustcp).
• Neither lane's findings fail the build going forward.
• docs/local-checks.md reflects the new advisory status and the
  release-prep sweep responsibility.

Out of scope

• Replacing IWYU outright — superseded by the conclusion above; the
  alternative-tool investigation goes back into E24's candidate ideas.
• Making cppcheck advisory — low false-positive rate; staying required.
• Adjusting branch-protection required checks — IWYU lanes report
  success going forward (via step-level continue-on-error), so the
  required-checks list does not need a UI edit.

Notes for the next pass at this

If we revisit a tool swap, key constraints we discovered:

• misc-include-cleaner's stdlib mapping covers ISO C cleanly, has gaps
  on POSIX extension macros and structs.
• It honours the existing // IWYU pragma: keep / : export /
  : private directives, so a swap would not require pragma rewrites.
• It folds into the existing per-TU clang-tidy compile step (via
  CMAKE_C_CLANG_TIDY / CMAKE_CXX_CLANG_TIDY), so no separate lane
  / preset is needed.
• The standalone clang-include-cleaner tool (not the clang-tidy
  check) does support IWYU-style mapping files and may close the
  POSIX gap, but that path was not explored in this story.