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>

wm/src/draw.c

@@ -146,10 +146,7 @@ void frame_draw(WM *wm, Client *c) {
         xft_color(wm, c->focused ? COLOR_TITLE_TEXT_ACT : COLOR_TITLE_TEXT_INACT, &col_text);
         XRectangle clip = { BORDER_W + 4, 1, (unsigned short)text_max_w, TITLE_HEIGHT - 2 };
         XftDrawSetClipRectangles(xd, 0, 0, &clip, 1);
-        XGlyphInfo ext;
-        XftTextExtentsUtf8(wm->dpy, wm->font_title,
-                           (const FcChar8 *)c->title, strlen(c->title), &ext);
-        int ty = (TITLE_HEIGHT - ext.height) / 2 + ext.height - ext.y;
+        int ty = (TITLE_HEIGHT + wm->font_title->ascent - wm->font_title->descent) / 2;
         XftDrawStringUtf8(xd, &col_text, wm->font_title,
                           BORDER_W + 6, ty,
                           (const FcChar8 *)c->title, strlen(c->title));

wm/src/menu.c

@@ -7,7 +7,7 @@
 static const MenuItem MENU_ITEMS[] = {
     { "App Launcher",      "orionlauncher &"                              },
     { "Terminal",          "alacritty &"                                },
-    { "File Manager",      "thunar &"                                   },
+    { "File Manager",      "rigel &"                                    },
     { "Text Editor",       "mousepad &"                                 },
     { "──────────────────", NULL                                         },
     { "Web Browser",       "firefox &"                                  },