Report a pc-joint entity reference that does not resolve

[willeastcott]

Fixes #389

An entity-a or entity-b that names nothing live produced no constraint and no message, so the joint simply did nothing — indistinguishable from a physics problem, and the failure most likely to bite when referencing a pc-node, whose entity does not exist until its container asset has loaded. The silence was deliberate and pinned by a test; this replaces it, because every neighboring element already reports the same class of mistake loudly: pc-model warns for an asset id that resolves to nothing, and pc-node warns for a name it cannot find, down to offering the closest match.

This is option (1) from the issue: warn on a non-empty reference that does not resolve, with no timing change and nothing breaking.

What warns, and how

The two failures report differently, because they have different fixes:

• Nothing in the document matches the reference — usually a typo or a missing element:

  pc-joint could not resolve entity-a '#nope' - nothing in the document matches it - constraint not created. Assign entity-a again once the entity exists.

• Something matches but is not backing an entity — usually timing (a pc-node whose asset has not loaded), or the wrong element entirely:

  pc-joint could not resolve entity-a '#shin' - <pc-node> matches it but is not backing an entity yet - constraint not created. Assign entity-a again once the entity exists.

Telling them apart needs the matched element, so the element lookup is split out of getEntity as findEntityElement (both @internal). getEntity keeps its signature and its silence — what an unresolved reference means depends on the element holding it, so reporting stays with the caller.

What stays silent

An empty reference. On entity-b it is the documented way to pin the first body to a point in world space, and on either attribute it is the transient state of an element whose reference has yet to be assigned. A test pins this.

Scope

pc-joint only — the most punishing of the five reference-resolving elements, because a missing joint has no visual symptom at all. pc-button, pc-scrollbar, pc-scrollview and pc-script are left alone here.

Test plan

• warns when a reference matches nothing in the document (replaces the test that pinned the silence)
• warns differently when a reference matches an element backing no entity
• warns again when a reference is reassigned and still does not resolve
• stays silent for an empty reference, which is the world-space case
• Full suite: 996 tests pass across 46 files; lint, type-check and build clean.

🤖 Generated with Claude Code