[v0.3] Replace cut slot with a real kill ring

[owainlewis]

Goal: Replace the current single cut/yank slot with a small Emacs-shaped kill ring.

Context: Cortex already has mark, region cut, C-k, and C-y in the local editor surface.
The PRD calls for a real ring with yank-pop behavior.
This is core editing feel, not a broad feature expansion.

Proposed approach:

• Store killed text in a bounded ring instead of one optional string.
• Push region kills and line kills onto the ring.
• Keep C-y yanking the most recent entry.
• Add yank-pop with M-y only immediately after a yank.
• Keep behavior simple when there is no prior yank: show a status message and leave the buffer unchanged.
• Preserve dirty state and point behavior across kill, yank, undo, and redo.

Acceptance criteria:

• Multiple kills are retained in newest-first ring order.
• C-y inserts the newest kill at point.
• M-y after C-y replaces the just-yanked text with the previous ring entry.
• M-y without a preceding yank reports a clear no-op status.
• Undo cleanly reverses kill, yank, and yank-pop edits.
• Region state is cleared after edits that should clear it.
• Existing kill line, kill region, save, search, picker, and quit behavior still works.

Verification:

• Run cargo test.
• Manually kill two different chunks, yank, yank-pop, undo, redo, save, and quit.
• Manually confirm terminal cleanup after exit.

Out of scope:

• System clipboard integration.
• Persistent kill history.
• Emacs-compatible kill coalescing across every edge case.
• Visual kill ring browser.