feat: generate compile database without building

[wellwei]

Summary

• add mcpp build --configure-only, reusing the real build plan and existing build selectors while skipping ordinary compilation, linking, BMI cache population, and build-success cache writes
• include tests/**/*.cpp, matching [build].flags, and test dev-dependencies in the generated CDB; stage only std and already-cached dependency BMIs needed by language tooling
• publish compile_commands.json atomically across platforms, preserve last-known-good files and CDB symlinks, and keep normal build/test publication failures non-fatal
• document the trust and side-effect boundary: configure-only may still execute build.mcpp, resolve/install dependencies or toolchains, and update lock/resolution metadata

This is the independently agreed A part from #379. It intentionally does not add JSON/NDJSON, ide subcommands, snapshots, mcpp.wire, invalidatedBy, or new selector semantics. It does not close the RFC.

Core impact

• shared test target discovery is extracted from mcpp test; existing list/build/run behavior remains covered by unit and E2E tests
• the Ninja backend gains an opt-in requireCompileDatabase flag; default build/test behavior remains warning-only
• CDB publication changes from direct truncation to same-directory temporary write plus atomic replacement; concurrent writers remain documented last-writer-wins

Test plan

• fresh self-hosted mcpp build --no-color --no-cache on macOS ARM64
• fresh mcpp test --no-color: 70 passed, 0 failed
• tests/e2e/202_configure_only_cdb.sh: syntax-error source, test/dev-dependency flags, workspace fan-out, -p, no build artifacts/cache, publication failure preservation
• adjacent E2E: 01, 18, 35, 76, 77, 90, 157, 159
• unit coverage for test discovery, configure prerequisites, atomic replacement, unchanged mtime, symlink preservation, unreadable/failed destinations

Refs #379
Refs mcpp-community/mcpp-vscode#5

[Sunrisepeak]

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