fix(compiler): align contract route conflict validation

[cssbruno]

Summary

• Allow repeated references to the same generated contract route when they match the appgen dedupe identity: kind, method, route, contract, and owner.
• Use the shared backend route path key for contract route conflict registration after strict path validation succeeds.
• Add regressions for duplicate identical command/query references and malformed contract paths that must be reported as contract_route_invalid instead of being silently conflict-skipped.

Review Feedback

Addresses Codex review comments on PR #286:

• Allow duplicate references to the same contract.
• Validate contract paths with backend normalization semantics.

Verification

• go test ./internal/compiler ./internal/source
• go test ./...
• go build ./cmd/gowdk

LLM Assistance

• Implemented by Codex from automated review feedback.