Elements with a bound instanceOf or glyph content are skipped instead of rendered

[nathanacurtis]

When an element's identity comes from a prop binding rather than a literal, the writer skips the element entirely — losing its styles, its layout row, and the backing prop.

Evidence

SpotlightIcon:

path	baseline	round-trip
anatomy.icon	{type: glyph}	(absent)
props.icon	{type: string, examples: [accessTime]}	(absent)
default.layout[0]	{root: [icon]}	"root"
variants[0..3].elements.icon	fill colour per variant	(absent)

Mark:

path	baseline	round-trip
anatomy.asset	{type: instance, instanceOf: AssetMarkBrandMemberOnlyDeal}	(absent)
props.name	{type: string, examples: [AssetMarkBrandMemberOnlyDeal]}	(absent)
default.elements.asset	instanceOf: {$binding: "#/props/name"} + sizing	(absent)
variants[0..8].elements.asset	sizing per variant	(absent)

Where — the fix already exists in this codebase

packages/figma-from-specs/src/:

• Elements/GlyphElement.ts — create() resolves a glyph name only from static content (typeof spec.content === 'string' or $value) and returns null for a $binding, so Elements.createChild skips the element.
• Elements/Elements.ts — expandCollapsedRoot() already solves exactly this for the ADR 058 collapsed-root case: it calls Element.resolveContentText(rootSpec, props) to resolve a bound content to the prop's first example before creating the leaf. The same resolution applied in createChild's glyph branch is the fix.
• Props/Props.ts — swapDefaultId() mints no INSTANCE_SWAP property when the target has no manifest entry, which is the Mark half.

Note Mark's target is a genuinely external asset mark, so the element should still render with its example rather than resolving to a library component.

Fixtures

• SpotlightIcon — bound glyph content
• Mark — bound instanceOf

Two components, both with aligned variant counts, both small.

Verification

Round-trip both. anatomy.icon / anatomy.asset survive, the layout row is preserved ({root: [icon]} not "root"), and the backing prop is still in props.

Part of #281.