circuit_unpacker: dummy-leaf support for non-power-of-two N

[anatgstarkware]

Summary

Adds support for N that isn't a power of two by right-padding hints.leaf_outputs internally with dummy_leaf_output() up to next_power_of_2(N), and truncating the returned Vec<Vec<Var>> back to N entries.

Dummy slots are indistinguishable from real leaves to compute_subtree_hash: their constant output is guessed into the circuit and Blake-bound just like any other leaf, so the prover cannot substitute different values without falsifying the root hash. The crate-level "Dummy leaves" doc and the Cairo-unpacker comparison table are updated to reflect the padding approach.

Covered by unpacker_n3_one_dummy (N=3, N_total=4, slot 3 dummy).

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This change is 

[cursor]

PR Summary

Medium Risk
Changes how recursive proof unpacking binds dummy tree slots (Blake constraints vs skipped/precomputed subtrees), which affects soundness of the circuits-DSL unpacker path; scope is limited to this crate and covered by new tests.

Overview
Enables non-power-of-two leaf counts in circuit_unpacker by right-padding hints.leaf_outputs with the constant dummy_leaf_output() up to N_total = next_power_of_2(N), running the existing perfect-tree Blake recursion on the padded slice, then truncating the returned Vec<Vec<Var>> to N real leaves.

The previous power-of-two-only path and documentation about precomputed dummy-subtree hashes (skipping recursion into dummy subtrees) are replaced: dummy slots are now ordinary leaves in compute_subtree_hash—guessed and Blake-bound like real leaves—so substitution would break the root hash binding.

Tests add unpacker_n3_one_dummy and extend expected_subtree_hash to pad off-circuit the same way as production code.

^{Reviewed by Cursor Bugbot for commit 3ddf623. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.56%. Comparing base (a62182d) to head (3ddf623).

Additional details and impacted files

@@                      Coverage Diff                      @@
##           anatg/unpacker_impl_test1     #359      +/-   ##
=============================================================
+ Coverage                      59.43%   59.56%   +0.12%     
=============================================================
  Files                             36       36              
  Lines                           5367     5366       -1     
=============================================================
+ Hits                            3190     3196       +6     
+ Misses                          2177     2170       -7     

Files with missing lines	Coverage Δ
crates/circuit_unpacker/src/lib.rs	90.47% <100.00%> (+10.78%)	⬆️

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