fix(forester): retain proofs after eligibility ends

[sergeytimoshin]

Summary

• keep the proof-result receiver alive after a forester loses eligibility
• cache proofs that finish late and reuse them in a later eligible slot
• defer duplicate per-tree proof work while that cache is warming
• cover successful and failed late-proof completion with unit tests

Why

The existing cache only drained results that were immediately available when eligibility ended. Proofs still running in the prover completed after the receiver was dropped, so they could not be retried later and the same queue work was recomputed.

Validation

• cargo test -p forester late_proof --lib
• cargo check -p forester
• cargo fmt -p forester -- --check
• git diff --check

Local devnet evidence

Validated with both devnet foresters using TRANSACTION_MAX_CONCURRENT_BATCHES=3. For amt2kaJA14v3urZbZvnc5v2np8jqvc4Z8zDep5wbtzx, one result was available at handoff and three arrived late; all four were retained. The next eligible forester sent the four cached instructions in 1,401 ms without regenerating them.

Transaction: 21oZcBeMdhWTMbFeCdqM5Y7miuF5gUx2z1m6prXE42Hk82DCSY225zaJoGZoKjLrHSdLmMCDRim6qYn4ndsfS2Sr

The queue advanced from start index 977251 to 978251. No forester processing errors were observed; the short indexer-root mismatch afterward was expected lag because on-chain state had already advanced.

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