feat: Complete modeline overhaul with custom segments and interactive features

Replaced mood-line with a custom, minimal modeline using only built-in
Emacs functionality to avoid native compilation issues.

**Architecture:**
- Named segment system using defvar-local for easy reordering
- Emacs 30 built-in right-alignment (mode-line-format-right-align)
- All segments marked as risky-local-variable for proper evaluation

**Features:**
- Color-coded buffer names (green=writeable, red=read-only, gold=overwrite)
- VC branch with git symbol (U+E0A0) and state-based coloring
- Position format: L:line C:col
- Help-echo tooltips on all segments
- Mouse click handlers for interactive actions
- String truncation in narrow windows (< 100 chars)
- Active-window-only display for branch and misc-info

**Interactive Actions:**
- Buffer name: mouse-1 = prev-buffer, mouse-3 = next-buffer
- Major mode: mouse-1 = describe-mode
- Git branch: mouse-1 = vc-diff, mouse-3 = vc-root-diff

**Bug Fixes:**
- Disabled async native compilation to prevent "Selecting deleted buffer" errors
- Fixed difftastic loading by changing :demand to :defer
- Abstracted buffer status colors to user-constants.el for reuse

Inspired by Prot's modeline design patterns.

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