Edit the selected field in context

[michael-lins]

Depends on #34. Keep in Backlog until selection is proven.

Problem

Editing should happen beside the selected field, while preserving the existing safe mutation and undo loop.

Scope

Support only the existing placeholder edit in the Customer partial. Option/Alt-click opens a contextual editor; a visible keyboard-accessible edit action offers the same behavior. Reuse resident ownership, structured mutations, Herb validation/diff and one-level undo. No AI, event editing or arbitrary attributes.

Acceptance criteria

• Option/Alt-click on a supported field opens an editor beside it in design mode.
• A visible keyboard-accessible action opens the same editor.
• Apply changes only the selected literal placeholder in the actual partial; the caller remains unchanged.
• The candidate reparses with Herb and produces the expected structural diff before the write.
• Undo restores exact original bytes and the browser value through the existing revision/change-ID checks.
• Stale, ambiguous or unsupported edits display an actionable error without overwriting source.
• Escape cancels an unsubmitted edit; Ruphino remains usable for mode and undo controls.

Verification

Write focused tests first and retain source-preservation and stale-undo coverage. In the real browser, edit a field, observe reload and the Herb diff, then undo and compare exact bytes of the partial and caller. Run mise run test and stop all verification services.