Model live checkbox and radio indeterminate state #457

Parent: #257

Proven gap

WebScene parses :indeterminate but the matcher rejects it. HTMLInputElement.indeterminate is not exposed or stored, checkbox activation cannot clear/restore it, and radio groups do not invalidate derived indeterminate state when live checkedness changes.

Focused implementation

• add non-reflected live input.indeterminate boolean state to the existing cold form-control record and both generated/legacy binding lanes;
• match checkbox indeterminate state from that live property;
• clear checkbox indeterminate state during click activation and restore it when activation is canceled;
• derive radio :indeterminate from the existing live checkedness group and target group members when checkedness changes;
• compile live/property and relevant checked/type/name dependency invalidation;
• paint the existing native checkbox affordance as a dash while indeterminate.

Deferred validation debt

This implementation-only slice intentionally skips browser/WPT/native correctness runs, binding checks, build, performance, allocation, memory, lifecycle, scene/pixel, product, hosted checks, and CI monitoring. Those remain required before #257 acceptance.

Complete form-owner/reparenting group semantics, shadow-tree edge cases, <progress>:indeterminate, range states, user-valid/user-invalid, and cumulative product acceptance remain outside this child. Consolidation PR #76 is untouched.