Fill a nested instance's non-default slot from its binding on render

[nathanacurtis]

Problem

A component's spec can consolidate a default-slot fill into its own layout tree while a non-default slot on the same nested instance stays bound to its own content entry (ADR-099). Render handles the first half and not the second: the consolidated tree is rebuilt correctly, and the bound slot is left empty.

The content is therefore lost on a round trip. Rendering such a spec and capturing the result back produces a spec with the binding and its entry gone, because there is nothing in the Figma file to capture.

Render reports nothing — the run succeeds with no warning.

Potential solution(s)

• Fill a nested instance's non-default slot from the entry its binding points at, the same way a component's own slot binding is filled
• Where the instance is itself content that arrived through a default-slot fill, resolve the binding against the host spec that carries it rather than the instance's own component, which render does not hold
• Warn when a binding names a slot the instance does not expose, rather than completing silently

Acceptance criteria

• A nested instance's non-default slot is filled from the entry its binding names
• A spec rendered and captured back keeps both the consolidated tree and the bound slot's content
• The default slot's consolidated children are unaffected
• A binding that cannot be resolved is named in a warning rather than passing silently

Case data

• Fixture: testSlotNonDefaultNested
• Workspace: specs-testing — round-trip testing
• Territory: figma-from-specs
• Size: m

Notes

Found by round-tripping the fix for #677, which closed the capture half. Capture now emits the correct shape:

layout: [root: [levelZeroText, level1: [twoSlots: [aChild]]]]
twoSlots:
  propConfigurations:
    aside: { $slotContent: "…/slotContentExamples/testSlotTwoSlots__aside__default" }

Rendering that and capturing it back returns the tree intact and no aside — the bound entry's text never reaches the canvas.

Worth noting for whoever picks this up: a spec copied away from its run metadata loses the naming convention it needs, and render then fails to resolve instance elements for an unrelated reason. Keep latest.metadata.yaml beside a spec folder when rendering one in isolation, or the symptom looks like this issue and is not.

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Implementation details are tracked internally.

[nathanacurtis]

Closing as a duplicate of #673, which covers this and more.

I filed this after round-tripping the #677/#672 fix and seeing the bound non-default slot render empty. #673 already has it, names the exact cause — object-valued prop configurations are skipped, documented as deliberate Phase 4 scope — and additionally covers $binding pass-through and $nested deep configs, which this issue did not.

The round-trip evidence belongs there, so I have added it to #673 rather than keeping a narrower issue alive beside it.