Add word, page, and line navigation for source editing

[owainlewis]

Add word, page, and line navigation for source editing

What are we building?

Movement is limited to characters and single lines. Add the navigation commands needed to reach code quickly with Emacs-style keys.

Why?

This removes a concrete obstacle to the local coding loop tracked in #160 while keeping Cortex small and predictable.

Done when

• M-f and M-b move by Unicode-aware words; M-d and M-Backspace kill forward/backward words through the kill ring.
• M-< and M-> move to buffer start/end, preserving grapheme boundaries.
• C-v/PageDown and M-v/PageUp move by the current visible page with a small overlap and keep point visible.
• goto-line accepts a positive one-based line number through M-x; invalid input leaves point unchanged with a clear message.
• All commands appear in the registry and have focused keymap/view/app tests for empty files, EOF, Unicode, and long lines.

How to check

cargo test view::; cargo test keymap::; cargo test app::; cargo fmt --check; cargo clippy --all-targets -- -D warnings; cargo test. Navigate and kill/yank words in a Unicode fixture, jump to a line, page through it, and exit.

Agent notes

• Depends on: [v0.3] Add command registry and M-x #46 and [v0.3] Replace cut slot with a real kill ring #47.
• Source: Deliver a minimalist daily coding editor #160 and the product direction in docs/prd.md. Keep macOS-only behavior, Ropey storage, view-owned point/scroll, and the renderer boundary. Preserve dirty-buffer, atomic-save, disk-change, and grapheme-boundary guarantees.
• Define words as Unicode alphanumeric runs plus underscore, traversing separators predictably. No structural AST navigation or configurable word classes.

[owainlewis]

Implementation: #183 at d2a912d.

Word, page, buffer-endpoint, and line commands now reach source locations directly from keys or M-x.
M-f/M-b move over Unicode words, M-d/M-Backspace cut through the kill ring, M- reach buffer endpoints, C-v/PageDown and M-v/PageUp move with two lines of overlap, and goto-line validates a positive line number and clamps beyond EOF.

Word movement retains grapheme context across rope chunks in both directions.
Review found that restarting Unicode context froze one word command for over six seconds on 16,000 flag emoji; the retained cursor reduces the same optimized probe to 3.24 ms forward and 7.77 ms backward.
The fix includes deterministic context-work bounds and comparisons with flat Unicode segmentation.
Paging stores the last rendered viewport height in View and preserves the preferred display column.

Verification: 398 unit tests pass (four existing performance diagnostics ignored), formatting, strict Clippy across all targets, and release build pass.
The release PTY session passed Unicode movement, word cuts/yank/undo, buffer endpoints, both page key forms, goto-line validation/EOF, save, and shell restoration.
Eleven of thirteen terminal integration tests pass locally; the two redirected-input tests encounter the same sandbox restrictions documented in prior PRs and remain unchanged for macOS CI.

[owainlewis]

Merged in #183 at eb69a9a.

Word movement/kills, buffer endpoints, overlapping page movement, and goto-line are delivered with Unicode and edge coverage.
Independent review approved after fixing repeated flag-emoji context scans: the 16,000-flag probe improved from about 6.3 seconds in each direction to 3.24 ms forward and 7.77 ms backward.
Formatting, strict Clippy, release build, 398 unit tests, and all 13 terminal tests pass in macOS CI.
The release PTY workflow passed navigation, kill/yank/undo, line jumps, save, quit, and shell restoration.

Final tracking: issue closed; Project item set to Done and archived next, with parent checklist reconciliation.