Harmonic Continuous Mode

[cavemanloverboy]

from devin:

Harmonic Continuous Mode Support

Adds support for continuous harmonic blocks evenly throughout a leader slot.

Changes

Block stage streaming (block_stage/mod.rs, scheduler_synchronization.rs)

The block stage can now process multiple block messages per slot. Key changes:

Introduces a begin_block_execution / end_block_execution mechanism so votes are only deferred while a block is actively executing, not for the entire slot.
Adds is_block_consuming to track whether block stage still holds the slot claim, separate from whether it is mid-execution.
After the delegation period ends (all messages received), the block stage restores the vote limit and releases the slot claim if we are not leader in the next slot, allowing vanilla/bundle scheduling to append non-vote transactions.
On a failed message, block stage halts for the remainder of the slot rather than reverting to vanilla mid-slot.
Removes block_failed revert-to-vanilla path, which is no longer appropriate under streaming.
Passes LeaderScheduleCache into block stage to check next-slot leadership.
PoH pacing removal (poh_service.rs)

Removes the special PoH tick pacing that previously extended tick timing while waiting for a block and sprinted through remaining ticks after receiving one. This behavior is no longer needed — tick pacing stays at the original target throughout.

Bugfix: CU accounting in add_executed_transaction_costs (cost_tracker.rs, block_consumer.rs)

The block consumer's add_executed_transaction_costs had a hand-rolled cost tracking implementation that incorrectly added loaded_accounts_data_size to allocated_accounts_data_size, inflating the allocation budget and blocking subsequent votes with WouldExceedAccountDataBlockLimit.

The fix replaces the manual accounting with the existing add_transaction_cost + update_execution_cost pipeline — estimate costs upfront, then immediately rebate to actual execution values. This reuses the same well-tested code path that the standard QoS flow uses, just skipping the would_fit check since the block is already committed.