starknet_patricia: collapse leaf handling to a single match

[yoavGrs]

Match on LeafSource exactly once in compute_filled_tree_rec: the ExistingLeaves read is
inlined and the helper (renamed compute_leaf) now only handles the ComputeLeaves case,
returning the leaf and its output directly instead of an Option.

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

[yoavGrs]

• starknet_patricia: collapse the let-else + re-match in node_from_edge_data into a single match #14623
• starknet_patricia: use OnceLock for write-once filled-tree output maps #14622
• starknet_patricia: assert unmodified-subtree binary child is present in the skeleton #14614
• starknet_patricia: extract finalize_binary_child helper from node_from_binary_data #14613
• starknet_patricia: thread the leaf output map separately from LeafSource #14600
• starknet_patricia: collapse leaf handling to a single match #14583 👈 (View in Graphite)
• starknet_patricia: introduce LeafSource enum for filled-tree leaf handling #14582
• main

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[reviewable-StarkWare]

This change is 

[cursor]

PR Summary

Low Risk
Internal refactor within Patricia filled-tree construction; behavior should be equivalent with clearer control flow and no API surface change.

Overview
Filled tree leaf handling in compute_filled_tree_rec is restructured so LeafSource is matched once at the leaf skeleton node instead of inside a shared helper.

get_or_compute_leaf is renamed to compute_leaf and narrowed to the ComputeLeaves path only: it takes the leaf input map and index, returns (L, L::Output) (no Option on the output). ExistingLeaves is handled inline in the UpdatedSkeletonNode::Leaf arm via L::from_modifications.

For ComputeLeaves, leaf output is written to leaf_index_to_leaf_output in that same arm right after compute_leaf, removing the follow-up if let that previously wrote outputs after hashing the leaf node.

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[nimrod-starkware]

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[yoavGrs]

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