Retry dusty CAT combines onto a remainder coin

[hoffmang9]

Summary

• After the two-sided spread, buy clips like 24,975 mojos left CAT dust on the tightest covering set (two 25,025 coins → 100 mojo change), so the shaper returned CannotFund even when a remainder coin could absorb legal change.
• Keep the 1 CAT dust floor. When a covering combine would be dust, retry once while skipping dusty overshoots so leftover change lands on an extra remainder coin, or on a different pair whose change is already legal.
• Fail closed when no covering set within combine_input_cap leaves legal change. Daemon flat combine (no dust context) is unchanged.

Test plan

• shape::combine tests: third-coin remainder, two 25.025s alone still rejected, alternate pair with legal change preferred
• selection test: skip dusty two-coin cover and take remainder (ignore dust fragments)
• Existing fragmented cap-5 / eco181 / bootstrap planner dust tests still pass
• After merge: rebuild john-deere binaries and restore the size-25 buy rung so bootstrap can mint 24.975 clips