• Add vim-style {N}G command to jump to a source code line number in the current file's diff. The target is the new-side line number shown in the gutter (i.e. the actual source line), not the display row in the TUI.
• Since diffs only show changed hunks, the requested line may not be present. When no exact match exists, the cursor jumps to the nearest visible diff line and a status bar message explains the approximation. If the file has no diff lines at all, a warning is shown.
• Digits accumulate in Normal mode and are displayed in the status bar (NORMAL 42) for visual feedback. Pressing G consumes the count; any other key discards it. Plain G without a count still goes to the last file, preserving existing behavior.

Test plan

• cargo test — all 237 tests pass (17 new)
• Open a diff, press 4 2 G — cursor jumps to source line 42 (or nearest) in current file
• Press G without digits — still goes to last file
• Press digits then a non-G key (e.g. j) — digits are discarded, key handled normally
• Status bar shows accumulated digits as they're typed
• Press ? — help popup shows the new {N}G entry