Carry each node's preprocessed root in PackedNode

[YairVaknin-starkware]

Every PackedNode::Composite carries the preprocessed_root of its node's proof (eight little-endian u32 words: the leaf circuit's for leaves, the multiverifier's for fold nodes; extracted via the new ExtractedProofData). The unpacker's hints stay role-agnostic this way: each node's root is looked up in the supported-roots trust list instead of being derived from the node's role, so admitting new circuit types is just appending to that list.

Stack (3/4): prove_leaf_from_files (#390) ← retarget canonical (#391) ← this ← goldens e2e (#388).

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This change is 

[cursor]

PR Summary

Medium Risk
Changes the serialized packed-output format and how roots are supplied to downstream verification; behavior is localized but any unpacker or golden expecting the old Composite shape must update.

Overview
PackedNode::Composite now includes an eight-word preprocessed_root on the wire (leaf circuit root for leaves, multiverifier root for fold nodes). Leaf and internal builders take that root explicitly; fold reduction pulls it from each fresh proof via ExtractedProofData and digest_bytes_to_words.

This lets the unpacker use each node’s own root against a supported-roots list instead of inferring roots from node role. root_packed.json shape changes; unit and e2e expectations were updated to match.

^{Reviewed by Cursor Bugbot for commit ae52f97. 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 65.59%. Comparing base (cde498b) to head (ae52f97).

Additional details and impacted files

@@                           Coverage Diff                           @@
##           yairv/retarget_canonical_to_leaf_bl     #392      +/-   ##
=======================================================================
+ Coverage                                65.47%   65.59%   +0.11%     
=======================================================================
  Files                                       45       45              
  Lines                                     5886     5906      +20     
=======================================================================
+ Hits                                      3854     3874      +20     
  Misses                                    2032     2032              

Files with missing lines	Coverage Δ
...ates/stwo_run_and_prove_recursive_tree/src/fold.rs	98.36% <100.00%> (+0.32%)	⬆️

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[OmriEshhar1]

@OmriEshhar1 reviewed 2 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on Gali-StarkWare).

[OmriEshhar1]

@OmriEshhar1 reviewed 2 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on Gali-StarkWare).

[Gali-StarkWare]

@Gali-StarkWare reviewed 2 files and all commit messages, and made 1 comment.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on YairVaknin-starkware).

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crates/stwo_run_and_prove_recursive_tree/src/fold.rs line 174 at r3 (raw file):

struct ExtractedProofData {
    preprocessed_root: HashValue<QM31>,
    root_words: [u32; N_RESERVED],

Why do we need both?

Code quote:

    preprocessed_root: HashValue<QM31>,
    root_words: [u32; N_RESERVED],

[Gali-StarkWare]

with question

@Gali-StarkWare made 1 comment.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on YairVaknin-starkware).