feat(kit): clear dirty state when form values are restored - #17086

Fixes #17072

form.fields.*.dirty() now clears when a value is restored to the form’s mounted baseline. This covers UI edits and programmatic nested updates while preserving touched state and the existing default-value behavior. Resetting or remounting the form establishes a fresh baseline.

With the same supported fixtures, pristine code passes 3 of the 15 selected browser cases and fails 12 dirty-state assertions; this change passes all 15. The original five requirement and guard cases improve from 2 passing to 5 passing.

Weco research trajectory includes the rejected attempts and their source snapshots. The final supported-fixture baseline and candidate are steps 4 and 5.

Local validation: formatting and lint passed; the full workspace type check passed with workspace concurrency set to 1; Kit unit tests recorded 779 passed, 0 failed and 110 pending; the 15 selected Playwright cases passed without retries, skips or flakes. Generated types and a minor feat: changeset are included. The full pnpm test suite was not run locally.

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