Yield to the defer agent per mesh assignment during instantiation

[nickdnk]

Same kind of performance related issue as #822 - if you have one very big object (say you are not the author and cannot easily split it), its instantiation can cause a stutter. Instantiation already yields to the frame budget per node, but a single node with many mesh assignments (multi-material meshes) populates them all in one go. This yields per assignment too, tunable by the existing frame budget logic.

Note: Everything below AI generated.

Problem

InstantiateSceneInternal honors the defer agent's frame budget between nodes (IterateNodes awaits BreakPoint() per node), but PopulateHierarchy populates all of a node's mesh assignments in one un-yielding callback. A node with many assignments (any multi-material mesh; CAD/archviz-style content routinely has dozens per node) blows the frame budget with no way for the defer agent to intervene.

Fix

Make the population callback Task-returning and yield DeferAgent.BreakPoint() after each mesh assignment — the same contract node iteration already follows, one level finer. The PopulateHierarchy profiler sample now wraps one assignment instead of a whole node, so captures attribute per assignment.

No public API change (the callback is a local function). Under UninterruptedDeferAgent, BreakPoint() is a no-op and behavior is byte-identical.

Measurements

Unity 6000.5.3f1, macOS (Apple Silicon), Mono development player, streaming scenes with high-node-count glbs (up to ~40 primitive clusters per node):

	before	after
worst PopulateHierarchy sample	70.5 ms	0.15 ms
worst instantiation-driven frame	76-95 ms	within the defer agent's budget

Behavior notes for review

• Instantiation was already non-atomic under a budgeted defer agent (per-node yields); this makes the granularity finer, so a multi-material object can render some submeshes a frame or two before the rest. Same class of intermediate state as before, one level deeper.
• Cancellation can now land between a node's assignments; callers already own partial-scene teardown on cancel, so no new obligation.
• Assignment order, meshNumeration, and the EndScene timing (where skins/animations bind) are unchanged.