spill: oversized result is sent to the model un-spilled when no single field exceeds the spill threshold

Bug

spill/register_callbacks.py — lines 208–225 and 264–265.

The spill plugin only considers a field spillable if it is strictly larger than spill_threshold = min(preview_bytes, cap // 2) (line 222). per_field_limit is derived from len(spillable) (line 225) and the commit is guarded by if total <= cap: (line 264).

A tool result composed of many medium-sized fields — each ≤ threshold, but summing over the cap — leaves spillable empty, per_field_limit == 0, no replacements built, and total never drops. The oversized dict goes straight to the model with nothing spilled, defeating the plugin's only purpose.

Repro (defaults cap=32768, preview_bytes=4096)

12 fields × 3500 bytes = 42,000 bytes → spill_threshold=4096, spillable=0, commit condition False → result untouched.

Suggested fix

When spillable is empty but fixed_bytes alone exceeds the cap, fall back to spilling (or shrinking) the largest fixed fields so something is actually reduced, rather than degrading to "do nothing".

Start in spill/register_callbacks.py at lines 208–225 and 264–265, then reproduce the 12-field case with cap=32768 and preview_bytes=4096. Ensure that when no field exceeds the spill threshold but the combined result exceeds the cap, the largest fixed fields are spilled or shrunk and the result sent to the model is within the cap.