S23.09: Document and render the Platform adapter headers, grouped by platform #606

Parent epic: #246

Goal

Extend the API reference beyond Core/Interface to the platform adapter headers
under Platform/*/Interface, documented to the same value-add-only contract bar
as S23.02 (#248), and presented so each platform reads as a distinct set.

Surfaced while reviewing the S23.02 mkdoxy output (PR #604): mkdoxy scans only
Core/Interface, so the platform interface headers do not render at all, and
the generated reference is framed in C++ terms.

Scope

78 headers across 10 platforms: Posix (19), Windows (19), LwipRaw (11),
PlusTcp (8), MbedTls (6), OpenSsl (6), FreeRtos (3), Atomics (2), FatFs (2),
PlusFat (2).

1. Doc-comment pass over every Platform/*/Interface/*.h to the S23.02 bar
   — contract only (ownership / lifetime / blocking / error-and-return
   semantics / pool + Null-fallback behaviour), self-evident symbols left bare,
   never restating the signature, verified against the implementation. See the
   value-add bar in the project memory (feedback_comments_add_value).
2. Render them — add the platform interface dirs to mkdoxy (src-dirs or
   per-platform projects) so they appear in the API reference.
3. Platforms visible as a set — each platform (Posix / Windows / LwipRaw /
   PlusTcp / MbedTls / OpenSsl / FreeRtos / Atomics / FatFs / PlusFat) should
   read as a distinct group. Open design point: the directory split may be
   enough, or we add per-platform nav sections / separate mkdoxy projects /
   Doxygen module grouping. Decide during implementation.
4. C-idiomatic reference (folded in here, not a separate story/PR) — drop
   the C++-oriented generated navigation (Namespaces / Classes / Class
   Hierarchy / class-members) so a C library reads as headers -> functions ->
   structs. OPTIMIZE_OUTPUT_FOR_C is already enabled; this is about the
   generated nav/overview pages, not the flag.

Done when

• Every Platform/*/Interface header has contract doc-comments (or is
  explicitly @internal).
• The platform headers render in the site, visibly grouped per platform.
• No C++-only navigation sections remain in the API reference.
• mkdocs build --strict is green (the docs-build CI job).

Not in scope

• Interface -> implementer -> consumer relationship visualisation (the third
  render concern; still under design, tracked separately when the approach is
  settled).