Ungated fixes extracted from harmony-one/harmony#5122

[Frozen]

Summary

This PR extracts the mixed-binary-compatible, fork-free fixes from #5122.

These changes do not depend on StrictStateValidationEpoch, do not change its activation epochs, and do not include any fork-gated behavior from #5122. They are intended to remain consensus-compatible while v1.2 and v1.3 validators operate together.

Included fixes

Severity	Area	Fix
Critical	Consensus / view change	Reject VIEWCHANGE senders that are not committee participants.
Critical	Cross-shard receipts	Reject nil merkle proofs and CXShardHashes lists shorter than ShardIDs; preserve legacy acceptance of trailing hashes.
High	Cross-shard receipts	Bound pending receipt header epochs to current + 1 and cap requeues.
High	Block storage	Copy CommitSig and CommitBitmap into the correct fields.
High	Rewards	Bounds-check AddReward against the delegation snapshot.
Medium	Staking / validator	Apply MaxBLSPerValidator before BLS proof verification.
Low	Block processing	Check that the parent block exists before reading its root.
Low	Cross-shard receipts	Handle a nil CXReceiptsProof receiver.
Low	Staking / validator	Prevent slot-key slice aliasing on validator creation.
Low	Node messages	Guard empty block-message payloads.
Low	Rewards	Remove the unreachable unsigned lower-bound comparison.
Low	Blockchain config	Return a copy from GetVMConfig.

Explicitly excluded

All rows marked Gated: Yes in #5122 remain excluded, including the gated EVM, delegation, reward, vote-power, receipt-import, and validator-state behavior changes. This PR contains no StrictStateValidationEpoch configuration change.

Two initially ungated changes are also excluded because they alter mixed-binary state-transition behavior and require coordinated activation:

• zero-total-effective-stake vote-power handling;
• zero-external-stake slashing handling.

Strict equality for ShardIDs and CXShardHashes is likewise deferred to a coordinated activation. This PR rejects only a shorter hash list, matching the legacy accepted-input boundary while replacing its out-of-range panic with an error.

Provenance and scope

• Base: current harmony-one/harmony:main after Reject abandoned branch hashes #5106 and Guard emergency recovery ViewID transitions #5107.
• Code and regression tests are the mixed-binary-compatible subset of Mainnet Patch: cross-shard, staking, rewards, consensus and the VM #5122.
• No vote-power or slashing state transition is changed by this PR.

Testing

Focused local verification on the current head:

go test -mod=readonly -count=1 ./consensus ./core ./consensus/votepower ./staking/slash

Additional compatibility checks:

• the direct onViewChangeSanityCheck membership test fails when the committee check is removed;
• the trailing-hash proof test passes against both the base implementation and this PR;
• shorter hash lists and nil merkle proofs return errors without panicking.