Fix note mode keymap cleanup, stable file keys, and sign ID collisions

[hay-kot]

Summary

• remove note-mode buffer keymaps on exit so normal mappings are restored when note mode is inactive
• normalize note file keys against repository root (or absolute path fallback) to avoid cwd-dependent lookup mismatches
• replace range sign ID arithmetic with monotonic unique sign IDs to prevent collisions in large files
• add regression tests for file-key normalization and note-mode keymap cleanup

Validation

• nvim --headless -u NONE -i NONE "+set rtp+=." "+lua require('diff-review.git_utils'); require('diff-review.note_actions'); require('diff-review.note_mode'); require('diff-review.note_ui')" +qa
• full test suite could not run in this environment because Plenary is unavailable and outbound network to fetch it is restricted