Add bounded selector actions with checked target identity

[wolfiesch]

Summary

• Split selector actions out of Add safe local extension deployment and health checks #54.
• Add unique CSS selectors for click, type, and fill across the public protocol and adapters.
• Preserve revision, sensitive-field, and Commit checks; execute against the checked backend node.
• Normalize and bound selector text on operation display paths.

Verification

Workspace typechecks, SDK build, extension action contracts, adapter contracts, and rendering regressions passed.

[omp-maintainer]

P2 — coherent public mutation-surface feature, so merge timing needs maintainer sign-off.
Blocking: selector resolution can cross a document replacement after the revision check and mutate the replacement page.
Maintainer call: after that race is closed, is selector-addressed mutation intended for the 2.0 surface?
Thanks for the scoped split and contract coverage.

[omp-maintainer]

blocking: act() checks expected_page_revision on line 569, then awaits DOM.getDocument/DOM.querySelectorAll here and executes the returned backend node without revalidating the document identity. If a navigation replaces the document during those awaits before the asynchronous tabs.onUpdated revision bump is observed, this selector resolves in the replacement page and performAction() mutates that new page; ref-addressed actions instead fail when their old backend node disappears. This violates the documented invariant that document replacement rejects rather than selects a new target. Capture/compare the page identity around selector resolution (and cover the in-flight replacement case) before executing or staging it.