Editor: Let Cmd+K open Add Link in a block

[epeicher]

What

In a block editor window, pressing ⌘K (Ctrl+K) on text opened the shell's command palette instead of the editor's Add Link popover. Now ⌘K in a block opens Add Link, and ⌘K anywhere else (sidebar, toolbar, other admin pages) still opens the shell palette.

Before	After

Why

The chromeless bridge claims ⌘K in capture phase and forwards it to the shell as os-palette-cycle. In WP 7.1 the editor canvas is always a nested iframe. Its rich-text Add Link shortcut handles ⌘K inside the canvas, then Gutenberg re-dispatches a clone of the keydown to the admin document, targeted at the IFRAME, with defaultPrevented already set. Core's own command palette bails on that flag ("avoid obscuring the effect of the preceding handler"). The bridge ignored it and claimed the key anyway.

Fix

• src/chromeless-bridge.js: the ⌘K forwarder returns early when the keydown is already defaultPrevented, the same rule Core's palette follows.
• docs/bridge-protocol.md: the keyboard-forwarders section documents that exception.

Testing

• tests/vitest/chromeless-bridge-links.test.ts: two new cases. An unclaimed ⌘K still posts os-palette-cycle; one the canvas already claimed does not. The second fails without the fix.
• Full suite: 536 files / 6,710 tests pass; tsc --noEmit clean; eslint clean (only the existing file-length warnings).
• Manual, on a local site with real key presses: open a post in an editor window, select a word in a paragraph, press ⌘K. Before: the shell palette opens. After: the Add Link popover opens. Click into the editor sidebar and press ⌘K: the shell palette still opens.