fix(simplex/batcher): handle weighted scheme assembly failure without stall

[Sertug17]

Closes #4409. Supersedes #4526.

Problem

try_construct_certificate panics and after #4526, stalls when Scheme::assemble returns None despite a participant-count quorum being met. This happens with weighted / threshold schemes (e.g. stake distribution [7, 1, 1, 1] with a share threshold of 8): the three minority participants verify, verified.len() >= quorum triggers try_complete(), which consumes the votes and sets state to Complete, then assembly fails because only 3 shares are available (not 8).

After that:

• Pre-fix(simplex): retry certificate assembly after quorum #4526: .expect("verified … quorum must assemble") → panic
• After fix(simplex): retry certificate assembly after quorum #4526: panic removed but state is still Complete, so the
  high-weight signer's vote arrives and is silently dropped → permanent stall
• should_verify() also gates on verified.len() < self.quorum and never
  re-admits pending votes, even when a higher-weight signer is waiting

Fix

Certification<V>

• Add assembly_failed: bool field
• Loosen try_complete() guard: allow re-attempts when assembly_failed is set, even if verified.len() < self.quorum
• Add revert_complete(votes): resets state from Complete back to Incomplete and sets assembly_failed = true
• Update should_verify(): return true when assembly_failed && !pending.is_empty(), bypassing the participant-count quorum gate

try_construct_certificate

• Remove all .expect(…) from assembly calls
• Clone votes before the offload closure so they can be restored on failure
• Call revert_complete(backup) when assembly returns None

Testing

cargo test -p commonware-consensus simplex::actors::batcher

96 passed; 0 failed

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