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_accordion.dart

@@ -36,7 +36,7 @@ class ClideAccordion extends StatelessWidget {
             padding: const EdgeInsets.only(left: 4, top: 10, bottom: 4),
             child: Row(
               children: [
-                ClideIcon(expanded ? PhosphorIcons.caretDown : PhosphorIcons.caretRight, size: 10, color: tokens.globalTextMuted),
+                ClideIcon(expanded ? PhosphorIcons.byName('caret-down') : PhosphorIcons.byName('caret-right'), size: 10, color: tokens.globalTextMuted),
                 const SizedBox(width: 6),
                 if (leading != null) ...[leading!, const SizedBox(width: 6)],
                 ClideText('$label · $count',