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>

Makefile

@@ -5,9 +5,9 @@
 #   make clean    — remove build artifacts
 #   make iso      — build the bootable ISO (requires Arch Linux + archiso)
 
-.PHONY: all wm deskbar launcher installer nebula orionpkg clean install iso
+.PHONY: all wm deskbar launcher installer nebula orionpkg skykit rigel clean install iso
 
-all: wm deskbar launcher installer nebula orionpkg
+all: wm deskbar launcher installer nebula orionpkg skykit rigel
 
 wm:
 	$(MAKE) -C wm
@@ -27,6 +27,12 @@ nebula:
 orionpkg:
 	$(MAKE) -C orionpkg
 
+skykit:
+	$(MAKE) -C skykit
+
+rigel:
+	$(MAKE) -C rigel
+
 clean:
 	$(MAKE) -C wm        clean
 	$(MAKE) -C deskbar   clean
@@ -34,6 +40,8 @@ clean:
 	$(MAKE) -C installer clean
 	$(MAKE) -C nebula    clean
 	$(MAKE) -C orionpkg  clean
+	$(MAKE) -C skykit    clean
+	$(MAKE) -C rigel     clean
 
 install: all
 	# binaries
@@ -43,6 +51,7 @@ install: all
 	install -Dm755 themes/orionwallpaper   /usr/local/bin/orionwallpaper
 	install -Dm755 nebula/nebula           /usr/local/bin/nebula
 	install -Dm755 orionpkg/orionpkg       /usr/local/bin/orionpkg
+	install -Dm755 rigel/rigel             /usr/local/bin/rigel
 	# GTK theme
 	mkdir -p /usr/share/themes
 	cp -r themes/OrionOS /usr/share/themes/