Visible warning for unrecognized modifier kinds

[colemancda]

Makes schema drift visible at the modifier level — the analog of the existing unknown-node diagnostic (which renders ⟨unknown node⟩ in red).

What changed

• A KNOWN_MODIFIER_KINDS set enumerating every kind some consumer handles (composeModifiers folds, plus the ones read by RenderText, RenderChild's environment, RenderEffects, and container branches).
• composeModifiers() now outlines any node carrying a kind outside that set in red, instead of silently dropping it.

Verification

• Emulator (Appearance screen): a demo view with a custom modifier emitting an unrecognized kind (notImplementedInInterpreter) gets a thin red outline, while every neighboring example (clipShape, overlay, background, …) renders cleanly — no false positives on known kinds.

The set is the one maintenance point: a new modifier kind must be added here, or it'll self-flag on first render — which is the intended safety net.