perf: Add DI container caching for faster CLI startup

[CybotTM]

Summary

Caches the compiled Symfony DI container to /tmp/guides-container-cache/ for faster CLI startup times.

Changes

• ContainerFactory: Cache compiled container using PhpDumper
  • Generates unique cache key from vendor directory path
  • Stores compiled container as PHP class file
  • Automatic invalidation when container configuration changes
  • Graceful fallback when container has uncacheable parameters

Performance Impact

Container compilation is skipped on subsequent runs, reducing startup time. Most beneficial for development workflows with frequent CLI invocations.

See Performance Analysis Report for detailed benchmarks.

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Related PRs

PR	Description	Status
#1287	Rendering caching layer	Independent
#1288	RST parsing optimizations	Independent
#1289	This PR - CLI container caching
#1291	Symfony 8 compatibility	✅ Merged
#1293	ProjectNode O(1) document lookup	Independent

All PRs can be merged independently in any order.

[CybotTM]

Closing this: the cache is never written, so the change delivers no speedup at all. GuidesExtension attaches a ProjectSettings object via addMethodCall('setProjectSettings', ...), and PhpDumper::dump() refuses any container that holds an object parameter; the resulting RuntimeException is swallowed by the catch (DIRuntimeException). Measured on the current head: after a full render /tmp/guides-container-cache exists and is empty, and container creation takes the same time as on main.

Independently of that, the read path require_onces PHP from a hardcoded world-writable /tmp directory, which I would not want to ship even once the write path works. Making the container dumpable — moving ProjectSettings into a service definition — is a separate and larger change; if I pick this up again it will be a new pull request on that basis.