feat(runtime): validate BackendPlan before install

[zzylol]

Summary

• validate materialization identity and window invariants
• reject routes that reference undeclared materializations
• validate before atomic plan installation
• preserve the previous active snapshot on rejection
• return HTTP 422 for semantically invalid plans

Validation

• cargo test -p control_plane backend_plan --lib (21 passed)
• cargo test -p data_plane hot_reload_backend_plan --lib (4 passed)
• git diff --check

Stack

Depends on #463.