refactor: migrate to cpp-core submodule and enforce pin in CI

[efvincent]

Summary

This PR completes the AoC refactor to consume cpp-core through a pinned git submodule and adds CI enforcement for the submodule pin contract.

What changed

• Added cpp-core as a submodule at third_party/cpp-core and pinned it to release tag v1.0.1.
• Reworked module build discovery in makefile to compile AoC modules plus core modules from the submodule source root.
• Removed vendored duplicated core modules from src/base/.
• Updated puzzle module debug-header includes to use core_debug.h via include path.
• Added submodule pin verification script: ci/verify_cpp_core_submodule.sh.
• Added make target: make verify-submodule-pin.
• Updated docs:
  • README.md with submodule setup, verification, and layout updates.
  • docs/CONTRIBUTING.md with submodule contract and bump workflow.
  • docs/Doxyfile input to include third_party/cpp-core/src.
• Added GitHub Actions workflow:
  • .github/workflows/verify-cpp-core-pin.yml
  • Runs make verify-submodule-pin on pull_request and push to main.

Validation

• make verify-submodule-pin
• make debug
• make release
• make lto
• make instrument
• ./build/aoc_worker_debug 2015 1 1
• make docs

Smoke output for day 1 part 1 remains unchanged (138).

Notes

• The submodule is intentionally pinned to a released semver tag, not a floating commit, per the downstream contract.