fix(cpp): extract declarations in guarded hh headers

[xiehuanyi]

Summary

C++ headers using .hh now select the C++ language in both the scanner and core registry. Structural extraction walks preprocessor conditionals, so classes, methods, functions and includes inside classic include guards or conditional branches are retained. Conditional branches are analyzed without evaluating compile-time flags, matching static analysis.

Linked issue(s)

Closes #710.

How I tested this

Added regressions for a guarded .hh header with a namespace/class/method/include, #if/#elif/#else function branches, and .hh scanner classification.

• pnpm lint
• pnpm --filter @understand-anything/core test — 47 files, 1,015 tests passed
• pnpm test — 39 files, 794 tests passed, 4 skipped
• pnpm build — all workspace packages built
• Scanner-focused suite — 41 tests passed

Versioning

• Version bumped from 2.9.7 to 2.9.8 in all six manifests listed in CLAUDE.md (the template still says five).

AI assistance: prepared with Codex and verified against the scanner, actual C++ grammar trees, and regression tests.

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