[M0-TOOL-01] Public API manifest: laige-api-scanner + checked-in laige-api.json + CI drift check

[offdev]

Scope (M0-TOOL-01, per roadmap/M0-foundations.md):

• tools/api/laige-api-scanner — single C++20 tool that scans the public headers (src/<module>/include for the PRD §10.1 modules) and emits the deterministic machine-readable manifest laige-api.json: every public symbol (class/struct/enum/function/method/constructor/destructor/variable/alias/enumerator/macro) with fully qualified name, header, line, normalized single-line signature, doxygen summary, @budget <text> annotation, and @experimental flag. Public-only access; namespace detail excluded; out-of-line member definitions, forward declarations, friend decls, and statements add no entries. The scanner is deliberately narrow (doxygen-comment-driven text walker, not a C++ parser) and fails loudly on unsupported constructs (CORE-008): block comments, raw strings, lambdas at declaration level, function-pointer parameter types, operator(), extern "C" blocks, compound typedefs, #define in class bodies. Full contract: the header comment of tools/api/laige-api.cpp.
• laige-api CMake target — canonical regeneration: cmake --build build --target laige-api rewrites the checked-in root laige-api.json (PRD §9.4, NFR-13.1). The checked-in manifest (376 symbols from 10 headers, all in laige-core — the other M0 modules have no include/ dirs yet) is committed in this PR.
• --check mode — byte compare (exit 0) or a symbol-level diff (added/removed/changed keyed by name+header+line, max 20 shown, parsed with laige::parseJson) on mismatch (exit 1); exit 2 on any error. Deterministic output (no timestamps, NFR-13.4) makes byte-identical regeneration the CI drift check.
• CI — new api-manifest job in ci-pull.yml/ci.yml: regenerates the manifest on every PR/merge and fails on drift, so adding a public symbol without regenerating fails CI. CTest (tests/api) runs the same check against the real tree in every P0 job (api-real-tree), plus fixture-tree tests: exact manifest bytes, fresh check, stale check with diff needles, unsupported-construct failure, root error.
• Docs in the same change (CORE-006) — docs/README.md index (DOC-001, previously missing), docs/getting-started/building.md status bullet (the canonical command was already reserved there), roadmap M0-TOOL-01 marked done with the decision record.

Key design decisions:

• One entry per public symbol: class-scope members under private:/protected: are excluded (struct=public, class=private defaults); everything inside namespace detail is excluded; a qualified member definition at namespace scope (Prng::seedState(...)) produces no entry — the in-class declaration is the canonical one (overloads stay distinguishable via line).
• Doc association: consecutive // lines directly above the declaration; blank comment line = paragraph break; banner lines (pure -=~* runs and --- section title --- headers) finalize without discarding; a physical blank line / closing brace / non-transparent directive discards the pending block (last block above a declaration wins); trailing // comments after code are wrapped continuations, never doc. Summary = first non-empty paragraph. @budget/@experimental tag lines fill their fields.
• The = of comparison operators (<=, >=, <=>) and of operator names (operator=, operator+=) is never an initializer; array-type [ after a template-id (ElementSlot<T>[]) is a type suffix, not a bound.
• laige-api-scanner is an executable and laige-api the custom target because CMake forbids two targets with the same name.

Incidental fix (latent since M0-CORE-08): isValidUtf8 in src/laige-core/json.cpp is [[maybe_unused]] — it is referenced only from debug asserts (fromString/setString UTF-8 preconditions), which compile out under NDEBUG, breaking the canonical Release build with -Werror=unused-function (CORE-010). The other build trees are Debug, which is why it went unnoticed.

Verification (commands run, all passing):

• cmake -S . -B build -DCMAKE_BUILD_TYPE=Release + cmake --build build -j + ctest — 22/22 (Release, GCC)
• same in build-asan, build-tsan, build-clang, build-shared, build-clang-shared, and a fresh build-clang-scratch — 22/22 each
• cmake --build build --target laige-api — regenerates the checked-in manifest byte-identically (md5 stable)
• ./build/bin/laige-api-scanner --root . --check laige-api.json — exit 0
• stale-fixture --check — exit 1 with + added: laige::E::B / ~ changed: laige::S (summary)
• block-comment fixture — exit 2; nonexistent root — exit 2
• two consecutive --out regenerations — byte-identical

Untested / remaining: MSVC (verified on the Windows CI job per convention — this PR adds no MSVC-specific code; the tool is POSIX-only in its file access via std::filesystem, like the existing tools). The 8 non-core modules contribute no headers yet; their manifests land with their include/ directories (the module list already mirrors tools/laige-include-lint).