Phosphor icons: resolve by name via a generated map (T-314)

Replace the 49 hand-maintained named consts with one generated
label→codepoint map (phosphor_glyphs.g.dart, 1512 glyphs from the glyph
table via tool/gen_phosphor_glyphs.dart). Feature code now references
glyphs by their exact kebab-case name — PhosphorIcons.byName('folder') —
with no raw codepoints; this also lets a Lua extension name an icon
without crossing the FFI boundary with a codepoint.

byName is total: an unknown name degrades to the `placeholder` box so the
bug is visible (it's a real error), while phosphor_glyphs_test asserts
every byName('...') literal in lib/ resolves — recovering the typo check a
const gave. Migrated the 89 call sites. Adds EmptyIconPainter for an
intentional blank that still reserves the icon box; ClideFilterBox gains
showIcon to keep the slot aligned when blank.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/widgets/src/clide_toast.dart

@@ -12,13 +12,13 @@ import 'package:flutter/widgets.dart';
 ({Color color, ClideIconPainter icon}) _styleFor(ToastSeverity s, SurfaceTokens t) {
   switch (s) {
     case ToastSeverity.success:
-      return (color: t.statusSuccess, icon: PhosphorIcons.checkCircle);
+      return (color: t.statusSuccess, icon: PhosphorIcons.byName('check-circle'));
     case ToastSeverity.warning:
-      return (color: t.statusWarning, icon: PhosphorIcons.warningCircle);
+      return (color: t.statusWarning, icon: PhosphorIcons.byName('warning-circle'));
     case ToastSeverity.error:
-      return (color: t.statusError, icon: PhosphorIcons.warningCircle);
+      return (color: t.statusError, icon: PhosphorIcons.byName('warning-circle'));
     case ToastSeverity.info:
-      return (color: t.statusInfo, icon: PhosphorIcons.circlesFour);
+      return (color: t.statusInfo, icon: PhosphorIcons.byName('circles-four'));
   }
 }
 
@@ -87,7 +87,7 @@ class _ClideToastState extends State<ClideToast> {
                     child: ClideTappable(
                       onTap: widget.onDismiss,
                       builder: (ctx, hovered, _) => ClideIcon(
-                        PhosphorIcons.xMark,
+                        PhosphorIcons.byName('x'),
                         size: clideIconStandard,
                         color: hovered ? tokens.globalForeground : tokens.globalTextMuted,
                       ),