Split knowledge watch state derivation

[yvette-carlisle]

Summary

• Move knowledge watch section/rebuild state derivation into watch/states.rs
• Leave rebuild/blocked output construction and candidate reasons in watch/outputs.rs
• Preserve the existing watch facade used by outcomes and candidate generation

Validation

• cargo make fmt-rust
• cargo make fmt-rust-check
• cargo make check-rust
• cargo make lint-vstyle
• git diff --check
• cargo make lint-rust
• cargo make test-rust (370 passed, 92 skipped)
• cargo make check (370 passed, 92 skipped)
• cargo make test-rust-integration (92 passed, 370 skipped)
• cargo make check-trace-gate
• Read-only scout and skeptic reviews