Correct evaluator and canonical replay contracts

[lanyue-llk]

Summary

• fix network-event evaluation for dynamic placeholders, ordered candidate streams, explicit last-event semantics, and typed array schemas
• correct canonical WebArena task contracts that previously described the wrong endpoint, payload, navigation mode, group set, repository, or route direction
• add positive and negative behavior controls, including trailing wrong requests and forward versus reverse route checks

Ownership

These changes belong in the lexmount/webarena-verified fork because they correct reusable evaluator and canonical dataset contracts. Browser replay execution, Rainbond deployment bindings, and run evidence remain in browser-eval.

Verification

• evaluator, migration, API, and dataset integrity: 1,862 passed
• complete locally runnable suite: 5,739 passed, 6,391 skipped, 2 deselected
• Ruff, ty, and diff checks passed

External-service and Docker-dependent tests were excluded from the locally runnable suite.

[lanyue-llk]

已将本 PR 的完整变更以普通 merge commit 收敛到 #3；#3 是唯一保留的 fork PR。