feat(beefy): allow non-consecutive validator set updates within a trusting period

[yrong]

Context

#1137 (comment)

Permit a commitment from a later session to be authenticated against the current validator set (a "skip-ahead"), instead of requiring a consecutive handover per session. This is gated to remain safe:

• canSkipAhead requires the id to be strictly ahead of the next set AND the era to be confirmed-stable (current.root == next.root), so signatures legitimately verify against the current root.
• A skip is only allowed while the current set is within trustingPeriod (14 days, < Polkadot's 28-day unbonding) measured by Ethereum's block.timestamp, so the set is provably still bonded and its honest-supermajority assumption holds. Past the window it reverts TrustingPeriodExpired.
• applySkip fast-forwards the current id (root preserved) and loads next from the leaf, but never refreshes currentSetActivatedAt, preventing a ratchet of the trust window. Genuine handovers re-anchor it, which keeps skips available across long eras (every era boundary forces a witnessed handover).

Wired into submitInitial, submitFinal, submitFiatShamir and createFiatShamirFinalBitfield.

Note: the relayer Go binding needs regenerating for the new currentSetActivatedAt and trustingPeriod getters.

[codecov]

Codecov Report

❌ Patch coverage is 80.55556% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.70%. Comparing base (161ae16) to head (1792528).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
contracts/src/BeefyClient.sol	80.55%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1802      +/-   ##
==========================================
- Coverage   78.90%   78.70%   -0.21%     
==========================================
  Files          24       24              
  Lines         986     1019      +33     
  Branches      187      197      +10     
==========================================
+ Hits          778      802      +24     
- Misses        185      191       +6     
- Partials       23       26       +3     

Flag	Coverage Δ
solidity	78.70% <80.55%> (-0.21%)	⬇️

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