feat: implement Phase 8 — headless tests, CI, and docs update

[konard]

Summary

Implements Phase 8 of the PMM visual demo development plan (tests, CI, documentation).

Fixes #7

What was done

New test files (Phase 8.1):

• tests/test_demo_headless.cpp — Smoke-test running all 7 scenarios for 2 s without a graphical window; verifies validate() == true and thread cleanup within 5 s
• tests/test_mem_map_view.cpp — Unit tests for MemMapView::update_snapshot(): null-safety, post-alloc/free snapshot rebuild, highlighted_block stability
• tests/test_scenario_manager.cpp — Lifecycle tests for ScenarioManager: count == 7, stop_all deadline, start/stop idempotency, destructor cleanup

Build system refactor (demo/CMakeLists.txt):

• Extracted pmm_demo_lib static library from demo sources (all files except main.cpp)
• pmm_demo executable links against pmm_demo_lib
• The three new headless test executables also link against pmm_demo_lib
• All three tests are registered with add_test() — run via ctest when PMM_BUILD_DEMO=ON

Documentation:

• docs/phase-8-tests.md — Phase 8 completion report
• plan.md — Marked all Phase 2–8 checklist items as ✅
• README.md — Added headless tests section with ctest usage instructions and updated repository structure

How to test

# Build with demo and run all tests
cmake -B build -DCMAKE_BUILD_TYPE=Debug -DPMM_BUILD_DEMO=ON
cmake --build build
ctest --test-dir build --output-on-failure

# Run only the new Phase 8 headless tests
ctest --test-dir build -R "test_demo_headless|test_mem_map_view|test_scenario_manager" --output-on-failure

Checklist

• clang-format passes on all new files
• cppcheck passes on all new files
• All new files ≤ 1500 lines
• plan.md updated — all phases marked complete
• README.md updated
• docs/phase-8-tests.md created

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[konard]

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💰 Cost estimation:

• Public pricing estimate: $2.980889
• Calculated by Anthropic: $2.576495 USD
• Difference: $-0.404394 (-13.57%)
  📎 Log file uploaded as Gist (2498KB)
  🔗 View complete solution draft log

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[konard]

✅ Ready to merge

This pull request is now ready to be merged:

• All CI checks have passed
• No merge conflicts
• No pending changes

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