circuit_unpacker: short-circuit fully-dummy subtrees

[anatgstarkware]

Summary

Replaces the "treat dummies as regular leaves" recursion with an off-circuit precompute of per-depth dummy-subtree hash constants, wired in-circuit as ctx.constants. When a subtree falls entirely in the dummy region (its leaf range starts at or after N), the recursion returns the cached constant directly — no Blake gate, no recursion, no guessed Vars on the dummy side. Mixed subtrees recurse only on the real side and combine with the cached dummy constant under one Blake gate.

API change

run_unpacker's preprocessed_root is now HashValue<QM31> (by value) instead of HashValue<Var>. The off-circuit precompute (precompute_dummy_hashes) needs the QM31 value; the unpacker wires it as ctx.constant internally, so callers no longer pre-wire pp_root. The existing tests are updated accordingly.

New test

unpacker_n5_dummies_short_circuit_at_two_depths — N=5, N_total=8: slot 5 is a single dummy leaf (depth-0 short-circuit) and slots 6,7 form an all-dummy depth-1 subtree (depth-1 short-circuit). Exercises both layers in one build.

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This change is 

[cursor]

PR Summary

Medium Risk
Changes how dummy padding is enforced in the Blake hash recursion for STWO proof unpacking; soundness relies on constants matching the root binding, but behavior is covered by new and updated tests.

Overview
Fully-dummy padding subtrees no longer recurse through dummy leaves with Blake gates and guessed outputs. The unpacker precomputes per-depth dummy-subtree hashes off-circuit (precompute_dummy_hashes), wires them as in-circuit constants, and compute_subtree_hash returns those constants when the subtree’s leaf range starts at or after N (no Blake, no dummy Vars). Mixed nodes still Blake-combine the real child with the cached dummy hash.

run_unpacker now takes preprocessed_root as HashValue<QM31> (by value); it internally constant-wires pp_root and the dummy-hash chain. Callers no longer pass pre-wired pp_root Vars. Output collection is sized for real leaves only.

Tests are updated for the API; unpacker_n5_dummies_short_circuit_at_two_depths covers depth-0 and depth-1 dummy short-circuits with N = 5, N_total = 8.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.99%. Comparing base (3ddf623) to head (bc08899).

Additional details and impacted files

@@                    Coverage Diff                     @@
##           anatg/unpacker_dummies     #360      +/-   ##
==========================================================
+ Coverage                   59.56%   59.99%   +0.43%     
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  Files                          36       36              
  Lines                        5366     5417      +51     
==========================================================
+ Hits                         3196     3250      +54     
+ Misses                       2170     2167       -3     

Files with missing lines	Coverage Δ
crates/circuit_unpacker/src/lib.rs	97.36% <100.00%> (+6.89%)	⬆️

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