feat(ide): add versioned project snapshots and pre-build CDB

[wellwei]

Summary

• add a versioned, read-only IDE project snapshot for workspace/member discovery, artifact state and structured diagnostics
• add an IDE configure operation that resolves the real build plan and publishes a clangd compile database before ordinary source compilation or linking
• define and document the IDE-client lifecycle while keeping workspace fan-out, freshness watching and stale fallback in the plugin

Closes #371

Protocol surface

mcpp ide snapshot --format json

• emits schema version 1 with protocol capability negotiation
• reports workspace members, package targets, selectors, CDB locations and aggregate partial/stale/configured state
• reports manifest and artifact failures as structured diagnostics
• performs read-only inspection with workspace path-containment checks
• treats configured as metadata plus regular-file existence, not active freshness or JSON validation

mcpp ide configure --format ndjson

• resolves the same BuildPlan and compile flags used by a normal build
• emits ordered operation-started, snapshot-published and operation-finished events with stable operation correlation
• publishes project, configuration and snapshot identities plus content-addressed reply CDB metadata
• includes discovered tests/**/*.cpp translation units and enables their development-dependency context
• stages standard-library and cache-hit module prerequisites needed by emitted compile commands
• publishes before compiling ordinary project TUs or linking final targets, so syntactically incomplete projects can still obtain clangd configuration
• closes the NDJSON lifecycle with a structured failure instead of mixing human progress into stdout

Publication and compatibility

• writes content-addressed replies under .mcpp/ide/replies/
• records the active configured snapshot in .mcpp/ide/current.json
• maintains root compile_commands.json as a compatibility projection for existing language-server clients
• serializes concurrent publication and distinguishes lock setup/I/O failures from actual lock contention in human diagnostics
• uses replacement-based fresh CDB publication on POSIX and Windows, retaining or restoring the previous usable root CDB on normal failures
• validates publication paths remain inside the physical project root

Build integration

• extracts shared test-target discovery so IDE CDBs and mcpp test use the same names, member scope and per-glob flags
• preserves best-effort mcpp test --list inventory for invalid manifests while real build/configure remains strict in prepare_build()
• keeps index-recovery notices off protocol stdout and updates the bootstrap pin used by this branch
• does not add a custom-toolchain feature or a new build backend

IDE client responsibilities and current limits

The new protocol documentation records that clients must:

• obtain workspace trust before configure, because resolution may install tools/dependencies, execute project or dependency build.mcpp, and update lock/cache/resolution state
• watch relevant inputs, debounce changes and avoid retriggering on files written by the active configure operation
• fan out workspaces through workspace.members[].workspacePath and one --package configure per member
• serialize operations per member, correlate seq/operationId, retain the previous usable CDB on failure and mark it stale
• treat the reply CDB as authoritative and not parse human error messages as stable categories
• represent missing uncached module BMIs as pending and offer an explicit normal build/test instead of silently building during discovery

Version 1 intentionally does not provide daemon mode, cancellation, configure --workspace fan-out, active freshness recomputation, a ready-artifact phase or cross-toolchain BMI compatibility guarantees.

Test plan

• self-build: mcpp build --no-color
• fresh binary: mcpp test --no-color (65 passed; 0 failed)
• tests/e2e/159_test_list.sh
• tests/e2e/196_ide_snapshot.sh
• tests/e2e/197_ide_snapshot_read_only.sh
• tests/e2e/198_ide_configure.sh
• tests/e2e/199_ide_configure_windows.sh on native Windows/MSVC CI
• git diff --check origin/main
• required GitHub Actions on Linux, macOS and Windows