apollo_propeller: rename shard_count to unit_count in should_build and should_receive

[sirandreww-starkware]

Note

Low Risk
Low risk: this is a parameter rename in should_build/should_receive with no behavioral change, only minor potential for downstream compile fixes if any callers relied on naming semantics.

Overview
Renames the should_build/should_receive threshold parameter from shard_count to unit_count in PropellerScheduleManager, aligning terminology with “units received” while keeping the same threshold logic.

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[cursor]

PR Summary

Low Risk
Parameter rename only; logic and thresholds are unchanged, so runtime risk is minimal beyond basic compilation/usage updates.

Overview
Renames the should_build/should_receive argument from shard_count to unit_count in PropellerScheduleManager to better reflect that these thresholds are based on received units (data/coding), not strictly shards. No behavior changes; call sites continue to pass received_units.len()/num_held_units unchanged.

^{Reviewed by Cursor Bugbot for commit cea441d. Bugbot is set up for automated code reviews on this repo. Configure here.}