apollo_propeller: rename shard_count/total_shards to unit_count/total_units

[sirandreww-starkware]

Note

Low Risk
Low risk rename-only change that updates variable/parameter names used to track held items during reconstruction and emit-threshold logic; no behavioral changes intended.

Overview
Renames reconstruction counter variables in MessageProcessor::handle_reconstruction_output from shard_count/total_shards to unit_count/total_units to better reflect that the code is counting units held (including an optional self-broadcast) when transitioning to PostConstruction.

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[reviewable-StarkWare]

This change is 

[guy-starkware]

@guy-starkware reviewed 1 file and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on noamsp-starkware and ShahakShama).

[cursor]

PR Summary

Low Risk
Purely a naming/clarity change in message reconstruction state transitions with no behavioral or protocol changes.

Overview
Renames reconstruction bookkeeping in message_processor.rs from shard_count/total_shards to unit_count/total_units, clarifying that the post-reconstruction held-count is tracking units (including a possible locally reconstructed+rebroadcast unit) rather than raw shards.

^{Reviewed by Cursor Bugbot for commit 1cabacf. Bugbot is set up for automated code reviews on this repo. Configure here.}