S10.20: Flip MISRA + naming gates to error mode and close E10

[DavidCozens]

Closes the E10 enforcement-mode flip and tidies the audit-phase artefacts that
were always slated for removal at the close-out. With S10.12–S10.19 done and
the latest main carrying 0 cppcheck-misra findings and 0 clang-tidy naming
warnings, the gates can flip without code work.

Bundle

1. cppcheck-misra → gating.
   .github/workflows/ci.yml analyze-cppcheck job — flip
   --error-exitcode=0 → 1 on both the "Run cppcheck-misra" step and the
   "Generate cppcheck-misra XML report" step.

2. Add Platform/MbedTls/Source/ to cppcheck-misra invocation.
   Append -IPlatform/MbedTls/Interface and Platform/MbedTls/Source/ to
   both cppcheck-misra invocations. Verification step only — the adapter was
   developed against the local warning-mode check (S08.07).

3. Widen the non-MISRA cppcheck XML pass.
   The diagnostic XML pass at line 511–522 is scoped to Core/Source/ only;
   widen to Core/Source/ + Platform/*/Source/ to match the cppcheck-misra
   scope. (Build-time cppcheck already gates Platform/ via CMAKE_C_CPPCHECK
   with --error-exitcode=1, but the artefact upload should report the wider
   surface.)

4. clang-tidy naming → gating.
   Root .clang-tidy — change WarningsAsErrors: '*,-readability-identifier-naming'
   to WarningsAsErrors: '*' and remove the soft-freeze comment block (lines
   34–37 reference the wrong story — S10.18 vs. S10.20 — so the easiest fix
   is to delete the now-stale paragraph entirely).

5. Delete docs/misra-conformance.md.
   Audit working doc, frozen since S10.12. Live truth lives in
   docs/misra-deviations.md + misra_suppressions.txt. No remaining
   cross-references in code or CI — only DEVLOG history references it.

6. Widen analyze-format to Platform/.
   Currently find Core/Interface Core/Source Tests Bdd/Targets …. Add
   Platform. (The epic lists this as "optional" — including it because
   E10's close-out is the natural moment, and format drift on Platform/
   has nothing to push it back.)

7. Mop-up nit from S10.22 PR refactor: S10.22 enum naming convention restoration (SCREAMING_SNAKE) #391.
   Tests/DatagramFake.h:18-19 comment says "calls beyond the configured
   slots return SOLIDSYSLOG_DATAGRAM_SEND_RESULT_SENT" — wording is
   ambiguous and reads wrong against the _FAILED fallback at
   DatagramFake.c:126. Re-word for clarity. (The other deferred nit was
   in misra-conformance.md, which gets deleted in item 5 — no fix needed.)

Acceptance

• All status checks green on the PR (cppcheck-misra and naming gates now
  exercised in error mode against a clean tree).
• git grep misra-conformance.md returns no matches outside DEVLOG.md.
• analyze-format step exercises a Platform/ file.

Re-proving the gates actually fail on a deliberate violation was considered
and skipped — the gates fired dozens of times during S10.07–S10.19, so
re-proving in this PR adds churn without information.

Out of scope

• No production code changes. No new deviations or suppression edits.
• The build-time cppcheck pass (CMAKE_C_CPPCHECK, --error-exitcode=1
  since the cppcheck preset landed) is unchanged.

Parent epic: #12