Add the typed requirement source model

[iperev]

Summary

• add a private, representation-neutral v2 requirement source model
• provide bounded deterministic admission, immutable projections, and exact metadata, reference, scenario, and relation semantics
• bind the model to REQ-PROOFKIT-SPEC-024 and close SOURCE-MODEL-01 while leaving codec and cutover work separate

Proof

• npm run check
• go test -race ./internal/kernel/requirementsourcemodel -count=1
• go vet ./internal/kernel/requirementsourcemodel
• go tool staticcheck ./internal/kernel/requirementsourcemodel
• 37 package tests bound to 37 selectors
• self coverage: 70/70 requirements, 198 scenarios

Non-Claims

• no source codec or persistence format is introduced
• no public CLI or public package API is added
• no existing requirement source is cut over
• no package or registry release is performed