feat(editor): retain cuts in a kill ring with yank-pop

[owainlewis]

Cortex now retains the last 32 nonempty cuts. C-y inserts the newest, and M-y or the named yank-pop command cycles through older entries, replacing only the previous yank. Movement, edits, save, buffer changes, and history commands invalidate that replacement.

Yank state checks buffer identity, revision, point, and the exact inserted character range. This preserves neighboring text when combining marks, regional indicators, or zero-width joiners make a yank merge with adjacent graphemes. Each kill, yank, and yank-pop remains independently undoable.

Validation: formatting, strict all-target Clippy, release build, 378 unit tests, and 11 of 13 terminal integration tests pass locally. The two existing redirected-input tests remain sandbox-restricted and require macOS CI. A release PTY session passed multiple line and region kills, C-y/M-y cycling, named yank-pop, undo/redo across saves, movement invalidation, and shell restoration. Fresh independent review approved with no findings.

Closes #47.

[greptile-apps]

Greptile Summary

This PR replaces the single cut slot with a process-local 32-entry kill ring and adds yank-pop cycling.

• Tracks the exact inserted character range, buffer identity, revision, point, and ring index for safe replacement.
• Adds Meta-y and named-command routing for yank-pop.
• Preserves independent undo boundaries for kills, yanks, and yank-pop operations.
• Adds focused coverage for ordering, eviction, invalidation, Unicode grapheme seams, buffer switching, and saved history baselines.
• Updates user and architecture documentation, although the [v0.3] Replace cut slot with a real kill ring #47 roadmap statuses remain stale.

Confidence Score: 4/5

The implementation appears behaviorally safe, but the roadmap must be updated to satisfy the repository’s documentation-alignment requirement before merging.

Kill-ring retention, yank-pop routing, stale-state validation, Unicode-aware replacement, and undo behavior are coherently implemented and tested; the only accepted issue is the stale status of issue #47 in two roadmap entries.

Files Needing Attention: docs/roadmap.md

Important Files Changed

Filename	Overview
src/app.rs	Adds kill-ring ownership, validated yank state, yank-pop replacement, invalidation paths, and extensive behavioral tests.
src/kill_ring.rs	Implements a newest-first deque retaining at most 32 complete nonempty cuts.
src/command_registry.rs	Registers yank-pop as a named editor command.
src/commands.rs	Adds YankPop to the command model and dispatch integration.
src/keymap.rs	Maps Meta-y to the new yank-pop command.
docs/roadmap.md	Closes issue #166 but leaves both entries for the newly delivered #47 feature marked open.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  K[Kill region or line] --> R[Push nonempty text to 32-entry ring]
  Y[C-y / yank] --> N[Insert newest entry]
  N --> S[Record buffer, revision, range, point, ring index]
  P[M-y / yank-pop] --> V{State still valid?}
  V -->|No| E[Report invalid yank-pop]
  V -->|Yes| O[Select next older entry]
  O --> X[Replace exact prior inserted range]
  X --> S
  C[Movement, edit, save, history, or buffer change] --> I[Invalidate yank state]

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