fix(cu): bind Electron text to clicked element

[hqhq1025]

Summary

• assign a document-local token to the exact editable DOM element focused by a semantic Electron click
• require the same connected, editable, active element before and after insert_text
• fail closed before dispatch when the click result lacks element identity or the document/element was replaced
• bound the page-side identity map to the latest focused element

This removes the old type-time elementFromPoint lookup, so same-page layout reflow cannot redirect text to a different field.

Verification

• built @maka/core, @maka/storage, and @maka/runtime serially
• npm --workspace @maka/computer-use test (128/128 passing)
• git diff --check

[Astro-Han]

Two P1 issues remain in the Electron text-target binding:

• P1: The normal new-document path calls __makaComputerUseReadElement before the page helper has been installed, so clicking an empty Electron field and typing can fail before insertion.
• P1: The incrementing element token is not bound to a document fingerprint or session. Reloading the same URL can reuse a token for another field, allowing an old session’s text to be inserted into the wrong target and reported as verified.

Please install/read the helper through one consistent path and bind tokens to the document identity plus session, invalidating them on navigation or reload.

[hqhq1025]

Superseded by #984, which consolidates #930-#933 into one target-identity boundary and addresses the requested changes. Stable AX token delivery is tracked in trycua/cua#2210.