Consolidate Rspack builder foundation

[anandtiwary]

What changed

• removes the legacy compatibility-layer Rspack package and duplicate example
• promotes the native implementation to @angular-builders/custom-rspack
• documents the Phase 2 supported/unsupported option contract
• rejects configured options that are not implemented instead of silently ignoring them
• adds unit coverage for option validation, schema metadata, and config merging
• adds package-archive and example/dev-server CI smoke checks

Why

The repository contained two competing implementations and both packages could report successful tests with zero test files. This change establishes one implementation and an honest, testable compatibility baseline before Angular compiler integration begins.

Validation

• package TypeScript build and schema generation pass
• 3 Jest suites / 17 tests pass
• packed package contains compiled builders, license, and compatibility documentation, with no source or test artifacts
• workspace discovery reports one Rspack package and one Rspack example

The example application runtime check is intentionally delegated to GitHub Actions because the local cached Rspack native library hangs in the current Codex runtime.