apollo_propeller: widen ShardIndex from u32 to u64

[sirandreww-starkware]

Note

Medium Risk
Changes a core identifier type used in shard creation/validation and protobuf serialization, which can impact wire compatibility and any downstream code assuming u32 indices. Main risk is subtle conversion/compatibility issues when interacting with older peers or stored data.

Overview
Widens ShardIndex from u32 to u64 across apollo_propeller, removing the prior narrowing conversions during shard creation and updating all call sites and tests accordingly.

Updates PropellerUnit protobuf conversions to treat index as a u64 end-to-end (dropping the previous u32 range-check/convert step) and adjusts Merkle-proof and reconstruction paths to convert from u64 to usize where needed.

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[reviewable-StarkWare]

This change is 

[sirandreww-starkware]

• apollo_propeller: replace local Stake type alias with StakingWeight from starknet_api #13037
• apollo_propeller: multiple shards per peer #12914
• apollo_propeller: changed CommitteeId to be a hash value #12913
• apollo_propeller: renamed Channel to CommitteeId #12911
• apollo_propeller: widen ShardIndex from u32 to u64 #12878 👈 (View in Graphite)
• apollo_propeller: e2e tests #12837 : 1 other dependent PR (#12838 )
• main

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[guy-starkware]

@guy-starkware reviewed 4 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on noamsp-starkware and ShahakShama).

[ShahakShama]

@ShahakShama reviewed 3 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on noamsp-starkware).

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• Started at 2026-03-19T16:24:46Z

[sirandreww-starkware]

@sirandreww-starkware reviewed 7 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on sirandreww-starkware).