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/builtin/git/src/extension.dart

@@ -20,7 +20,7 @@ class GitExtension extends ClideExtension {
           id: 'git.panel',
           slot: Slots.sidebar,
           title: 'Git',
-          icon: PhosphorIcons.gitBranch,
+          icon: PhosphorIcons.byName('git-branch'),
           titleKey: 'tab.title',
           i18nNamespace: id,
           priority: -80,

lib/builtin/git/src/git_status_item.dart

@@ -190,7 +190,7 @@ class _BranchPickerState extends State<_BranchPicker> {
                     onTap: () => widget.onDismiss(),
                     child: MouseRegion(
                       cursor: SystemMouseCursors.click,
-                      child: ClideIcon(PhosphorIcons.xMark, size: 12, color: tokens.globalTextMuted),
+                      child: ClideIcon(PhosphorIcons.byName('x'), size: 12, color: tokens.globalTextMuted),
                     ),
                   ),
                 ],