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>

skykit/skykit.h

@@ -0,0 +1,91 @@
+/* skykit.h — OrionOS native UI toolkit
+ * Shared palette, drawing utilities, and app boilerplate used by all OrionOS
+ * shell components (Rigel, Nebula, OrionPkg, installer, etc.).
+ */
+#pragma once
+
+#include <X11/Xlib.h>
+#include <X11/Xutil.h>
+#include <X11/Xatom.h>
+#include <X11/keysym.h>
+#include <X11/Xft/Xft.h>
+#include <fontconfig/fontconfig.h>
+#include <string.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+/* ── Palette ─────────────────────────────────────────────────────── */
+#define SK_BG         0x0E1220UL
+#define SK_SURFACE    0x141828UL
+#define SK_SIDEBAR    0x090D1AUL
+#define SK_PANEL      0x111828UL
+#define SK_BORDER     0x1A2840UL
+#define SK_SEP        0x162030UL
+#define SK_ACCENT     0x1E58A8UL
+#define SK_ACCENT_HOV 0x2A70CCUL
+#define SK_BTN        0x182848UL
+#define SK_BTN_HOV    0x1E3860UL
+#define SK_SEL        0x1A3058UL
+#define SK_ROW_ALT    0x0C1520UL
+#define SK_ROW_HOV    0x132030UL
+#define SK_TEXT       0xC8D4E8UL
+#define SK_TEXT_HDR   0xE8F4FFUL
+#define SK_TEXT_DIM   0x5A6880UL
+#define SK_TEXT_HOV   0xF0F8FFUL
+#define SK_TEXT_OK    0x44BB77UL
+#define SK_TEXT_ERR   0xFF5555UL
+#define SK_TEXT_WARN  0xFFCC44UL
+
+/* ── Standard fonts ──────────────────────────────────────────────── */
+#define SK_FONT_UI   "Sans:size=9"
+#define SK_FONT_SM   "Sans:size=8"
+#define SK_FONT_HDR  "Sans:size=10:weight=bold"
+#define SK_FONT_MONO "DejaVu Sans Mono:size=8"
+
+/* ── App context (one per process) ──────────────────────────────── */
+typedef struct {
+    Display  *dpy;
+    int       screen;
+    Window    root;
+    Visual   *vis;
+    Colormap  cmap;
+    int       depth;
+    XftFont  *font_ui;
+    XftFont  *font_sm;
+    XftFont  *font_hdr;
+    XftFont  *font_mono;
+} SKApp;
+
+/* ── Painter (per-window drawing context) ────────────────────────── */
+typedef struct {
+    SKApp    *app;
+    Drawable  drw;
+    GC        gc;
+} SKPainter;
+
+/* ── Simple button descriptor ────────────────────────────────────── */
+typedef struct {
+    int         x, y, w, h;
+    const char *label;
+    int         id;
+} SKBtn;
+
+/* ── API ─────────────────────────────────────────────────────────── */
+void          sk_app_init  (SKApp *app);
+void          sk_app_free  (SKApp *app);
+
+unsigned long sk_color     (SKApp *app, unsigned long rgb);
+void          sk_xft_color (SKApp *app, unsigned long rgb, XftColor *out);
+
+void sk_fill        (SKPainter *p, unsigned long rgb, int x, int y, int w, int h);
+void sk_rect        (SKPainter *p, unsigned long rgb, int x, int y, int w, int h);
+void sk_line        (SKPainter *p, unsigned long rgb, int x1, int y1, int x2, int y2);
+void sk_text        (SKPainter *p, XftFont *font, unsigned long rgb, int x, int y, const char *s);
+void sk_text_clip   (SKPainter *p, XftFont *font, unsigned long rgb, int x, int y,
+                     const char *s, int cx, int cy, int cw, int ch);
+int  sk_textw       (SKApp *app, XftFont *font, const char *s);
+int  sk_baseline    (XftFont *font, int y, int h);
+int  sk_in_rect     (int mx, int my, int x, int y, int w, int h);
+
+void sk_draw_btn    (SKPainter *p, SKApp *app, XftFont *font, const SKBtn *b,
+                     int hovered, unsigned long bg, unsigned long bghov);