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_lightbox.dart

@@ -114,7 +114,7 @@ class _ClideLightboxState extends State<ClideLightbox> {
             Positioned(
               top: 8,
               right: 8,
-              child: _IconChip(icon: PhosphorIcons.xMark, label: 'close', onTap: widget.onDismiss, tokens: tokens),
+              child: _IconChip(icon: PhosphorIcons.byName('x'), label: 'close', onTap: widget.onDismiss, tokens: tokens),
             ),
             Positioned(
               bottom: 8,