refactor(ui): modularize explorer, views, and merge editor

[esmuellert]

Summary

Split the largest UI workflows into responsibility-based directory modules while keeping their existing façades and user-visible behavior intact.

Changes

• Move Explorer refresh serialization and watcher/polling lifecycle into explorer/refresh/scheduler.lua
  • keep every public refresh request behind the same queue
  • serialize direct refreshes even when automatic refresh is disabled
  • preserve force coalescing and completion callbacks
• Split side-by-side views into façade, creation, update, and single-file modules
• Split inline views into façade, creation, update, rendering, single-file, buffer, and keymap modules
• Consolidate conflict-editor ownership under ui/conflict/
  • merge alignment, filler calculation, and auto-merge under conflict/merge/
  • input and Result presentation under conflict/view/
  • block, whole-file, combination, replacement, and diffget operations under conflict/resolution/
• Add module-loading and façade-delegation coverage
• Add behavioral protection for conflict resolution, dot-repeat, navigation, signs, and Result placement

Benefits

• Makes module ownership visible from the directory tree
• Reduces merge-conflict pressure in previously large single files
• Keeps stable façades for callers while allowing each workflow to evolve independently
• Places merge-editor calculations, presentation, and resolution under one conflict domain

Verification

• CODEDIFF_TEST_JOBS=4 ./tests/run_tests.sh — 107 spec files passed
• tests/module_loading_spec.lua — 48 passed
• tests/ui/merge_alignment_spec.lua — 15 passed
• Conflict resolution, navigation, signs, and layout specs — 9 passed
• AST/function-body comparisons confirmed moved implementations remained unchanged, including 47 conflict-editor functions
• Façade delegation and critical behavior were mutation-tested
• stylua --check passed
• git diff --check passed
• scripts/find_orphan_comments.py --rev main..HEAD found no orphaned comments