docs, bump Doxygen reference to v1.12.7 and drive Doxygen link warnings to zero

[steven-varga]

Follow-up to #282 (merged). The Doxygen reference still reads v1.12.6 and the build emits 327 warnings.

Scope

1. Selective version bump v1.12.6 → v1.12.7
   • Bump the h5cpp library references (download links [100–105], rel-h5cpp, "What's in", usability/handle-inventory "as of", DoxygenLayout, scorecards).
   • Keep h5cpp-compiler at v1.12.6 — it has no 1.12.7 release (links [200–206], rel-compiler, type-system-map) would 404.
   • Keep genuinely historical labels ("ROS3 resolved as of v1.12.6").
2. Correct h5cpp-multithreading-pipeline-state.md per fix, carry per-file worker pool in a fileno-keyed singleton registry (H5Fget_access_plist strips FAPL props) #286 — the FAPL worker-pool / parallel-decompress path is inert (falls back to synchronous); link fix, carry per-file worker pool in a fileno-keyed singleton registry (H5Fget_access_plist strips FAPL props) #286.
3. Remove 18 dangling \refs to 6 report pages dropped in the docs, integrate workspace reports + complete Doxygen alias system into one DRY Doxygen reference #282 reports-tree flatten:
   reports_type_system_architecture_notes, reports_performance_comparison_framework, reports_vs_h5py_syntax, reports_threaded_pipeline_sigma_queue, reports_reflection_cpp26_roadmap, reports_fapl_multithreading_workplan.
4. Drive all Doxygen warnings to zero — 133 unresolved "explicit link request" (auto-linked identifiers + cross-repo h5cpp-compiler symbols; likely exclude compiler-* reports from this project's Doxygen INPUT), and ~58 doc-comment @param/</tt> issues on h5::write/h5::create.

Acceptance

• doxygen Doxyfile (H5CPP_DOXYGEN_VERSION=1.12.7) produces 0 warnings.
• No 404 links; compiler links remain valid at v1.12.6.
• MT pipeline report accurate for v1.12.7 (fix, carry per-file worker pool in a fileno-keyed singleton registry (H5Fget_access_plist strips FAPL props) #286).

[steven-varga]

Superseded — the doxy v1.12.7 + zero-warning work was consolidated onto the authorized #282 branch (PR #284) as commit 3b61a8f. Created without authorization; removing.