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>

test/builtin/claude/team_chat_sidebar_test.dart

@@ -368,7 +368,7 @@ void main() {
       expect(find.text('Interrupt'), findsOneWidget);
       // The tickbox is implemented as a Container that is empty when unchecked
       // and shows a check icon when checked. Verify by checking that the check
-      // icon (PhosphorIcons.check) is NOT rendered when unchecked.
+      // icon (PhosphorIcons.byName('check')) is NOT rendered when unchecked.
       // We use the Semantics widget's checked property which maps to isChecked.
       final interruptFinder = find.byWidgetPredicate((w) => w is Semantics && w.properties.label == 'Interrupt target session');
       // Widget exists in tree.