feat: add MPF exclusion proofs with Aiken parity

[paolino]

Summary

Part of #149, based on the mpf-write extraction in #147.

This PR adds first-class MPF exclusion proofs and then fixes the remaining Aiken parity gap so the Haskell proof codec matches the canonical upstream off-chain implementation byte-for-byte.

What Changed

1. MPF exclusion proofs are now a first-class API

• Added MPF.Proof.Exclusion with:
  • MPFExclusionProof
  • exclusion proof generation for absent keys
  • pure exclusion verification
  • projection back to shared proof steps for Aiken/JS transport
• Re-exported the exclusion API from MPF.
• Added pure test helpers in MPF.Test.Lib.

2. Property parity with CSMT

• Added exclusion properties in MPF.Proof.ExclusionSpec:
  • absent key proves
  • present key returns Nothing
  • tampered target fails
• Added inclusion property parity in MPF.Proof.InsertionSpec:
  • random facts in a full tree
  • random facts in a sparse tree
  • deleted facts reject verification

3. Aiken parity fix

• Fixed fork-neighbor prefix encoding in MPF.Hashes.Aiken.
• The bug was that fork prefixes were encoded as packed nibble pairs; upstream encodes them as one byte per nibble.
• Added a peach[uid: 0] regression vector in MPF.ProofCompatSpec because mango and kumquat did not exercise that codec shape.
• Kept the pinned melon exclusion parity vector there as well.

4. Spec and handoff updates

• Added the issue spec artifacts:
  • spec.md
  • plan.md
  • tasks.md
• Updated the existing handoff note at exclusion-proof-handoff.md so it points at #149 and records the proof/property/parity status.

Verification

Passed locally:

• nix develop --quiet -c just test 'MPF.Proof.Exclusion'
• nix develop --quiet -c just test 'MPF.Proof.Insertion'
• nix develop --quiet -c just test 'MPF.Hashes.Aiken'
• nix develop --quiet -c just test 'MPF.ProofCompat'

I also compared our rendered CBOR against the canonical upstream off-chain implementation at aiken-lang/merkle-patricia-forestry HEAD (5ca6f3feaf9ab1e20007f164882dae3d2359745e) for 31 cases:

• 30 fruit inclusion proofs
• 1 melon exclusion proof

After the fork-prefix fix, that external comparison reported checked: 31, mismatches: [].

Reviewer Focus

• Proof design and verifier shape in MPF.Proof.Exclusion
• Byte-level codec change in MPF.Hashes.Aiken
• Upstream parity coverage in MPF.ProofCompatSpec

Follow-up

This does not yet wire MPF exclusion through MTS.Interface / MPF.MTS or the browser ptype = 1 path from #146. This PR is the proof-layer blocker removal needed before that routing work.