Phase 4: SkyKit library + Rigel file manager; fix text centering

SkyKit (skykit/):
- Shared palette constants, font setup, and Xlib/Xft drawing utilities
- sk_fill, sk_rect, sk_line, sk_text, sk_text_clip, sk_textw, sk_baseline
- Builds as libskykit.a, consumed by Rigel and future shell components

Rigel (rigel/):
- Native C/Xlib file manager with SkyKit
- Sidebar with 8 bookmarks (Home, Desktop, Documents, Downloads, etc.)
- Editable path bar (Ctrl+L or '/' to activate, Enter to navigate)
- Sortable columns: Name / Size / Modified (click header to sort/reverse)
- Colored type badges: DIR, IMG, SND, VID, ZIP, SRC, PDF, TXT, EXE
- Double-click to open dirs or files (xdg-open)
- Keyboard: ↑↓ select, Enter open, Backspace back, Ctrl+H toggle hidden
- Right-click menu updated: "File Manager" now launches rigel

Text centering fix (deskbar + wm):
- Root cause: draw_str added glyph-specific ext.height/ext.y offset, and
  all callers used hardcoded (H-10)/2 magic numbers instead of font metrics
- Fix: draw_str now takes y as a direct baseline; str_baseline() computes
  (strip_y + strip_h + ascent - descent) / 2 for consistent centering
- WM titlebar: same fix using font_title->ascent/descent
- Affects: "Orion ▾" button, clock, date, app list, flyout menu items,
  letter icons, and all window titlebars

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/docker/entrypoint.sh

@@ -238,6 +238,10 @@ install -m755 "$SRC/installer/orioninstall"        "$AIROOTFS/usr/local/bin/orio
 if [[ -f "$SRC/installer/orioninstall-gui" ]]; then
     install -m755 "$SRC/installer/orioninstall-gui" "$AIROOTFS/usr/local/bin/orioninstall-gui"
 fi
+# Rigel file manager (compiled binary)
+if [[ -f "$SRC/rigel/rigel" ]]; then
+    install -m755 "$SRC/rigel/rigel" "$AIROOTFS/usr/local/bin/rigel"
+fi
 
 # ── 5. GTK theme ──────────────────────────────────────────────────────
 echo ">>> Installing GTK theme..."