feat(firewood/syncer): implement change proof support

[rkuris]

Why this should be merged

The firewood syncer backend previously stubbed out the change-proof side of the sync.DB interface (placeholder type struct{}, marshaler/methods returned "not implemented"). Range proofs were the only working path, so the syncer fell back to full range proofs whenever a change proof would have been more efficient. Firewood v0.5.0 exposes the full change-proof API ((*Database).ChangeProof, .VerifyChangeProof → *Proposal, (*Proposal).CommitWithRebase, (*ChangeProof).FindNextKey), so this change lights up the real implementation end-to-end.

How this works

• proof.go: add a ChangeProof wrapper around *ffi.ChangeProof that holds the *ffi.Proposal produced by VerifyChangeProof until commit time; replace the stub marshaler with the real one.
• syncer.go: switch the type parameter from struct{} to *ChangeProof on *database and the New/newWithDB constructors; implement GetChangeProof (server side), VerifyChangeProof (client side, returns a Proposal), and CommitChangeProof (Proposal.CommitWithRebase + FindNextKey for the next-key continuation, matching the existing range-proof pattern).
• evm_syncer.go: implement (*evmDB).CommitChangeProof to enqueue code hashes from the proof before committing, mirroring the range-proof flow. (*ffi.ChangeProof).CodeHashes() is currently a no-op in firewood; the iterator loop is kept so we pick up real hashes automatically once it is implemented.
• handler.go: change-proof handler type params.
• External callers (graft/evm/sync/evmstate/firewood_syncer.go, syncer_test.go) updated to the new *ChangeProof type parameter.

Firewood version bumped to v0.5.0 in `go.mod`. v0.5.0 contains several correctness fixes that this code depends on (no-op proposal handling, `FindNextKey` returning nil at trie right edge, range-proof boundary checks).

How this was tested

Ran the firewood syncer test suite against a local firewood build at the v0.5.0 source. All 11 sub-tests pass:

• `Test_Firewood_Sync` (7 cases — range-proof sync from empty/non-empty client to various server sizes up to 100k keys)
• `Test_Firewood_Sync_UpdateSyncTarget` (4 cases — sync target update mid-flight, exercising the real change-proof path including `partial_sync_*_then_update` scenarios)

CI will fail until firewood v0.5.0 is published; reviewers can validate locally by pointing the firewood dependency at a v0.5.0 build via `go mod edit -replace`.

Need to be documented in RELEASES.md?

No.

Why this should be merged

How this works

How this was tested

Need to be documented in RELEASES.md?

[Copilot]

Pull request overview

Implements real change-proof support in the firewood syncer backend, replacing the previous struct{} stub with a *ChangeProof type that wraps *ffi.ChangeProof and the verification *ffi.Proposal. Bumps the firewood FFI dependency to v0.5.0 which provides the required APIs.

Changes:

• Add ChangeProof wrapper type with real marshal/unmarshal and implement GetChangeProof/VerifyChangeProof/CommitChangeProof on *database.
• Switch the type parameter from struct{} to *ChangeProof across database, evmDB, handler constructors, and external callers; implement evmDB.CommitChangeProof to enqueue code hashes before commit.
• Bump github.com/ava-labs/firewood-go-ethhash/ffi to v0.5.0.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
go.mod	Bumps firewood FFI to v0.5.0 (required for change-proof APIs).
database/merkle/firewood/syncer/proof.go	Adds ChangeProof wrapper and real marshaler implementation.
database/merkle/firewood/syncer/syncer.go	Implements GetChangeProof/VerifyChangeProof/CommitChangeProof; switches generics to *ChangeProof.
database/merkle/firewood/syncer/evm_syncer.go	Implements evmDB.CommitChangeProof (code-hash enqueue + commit) and updates generics.
database/merkle/firewood/syncer/handler.go	Updates handler constructors to new type parameter.
database/merkle/firewood/syncer/syncer_test.go	Updates syncer type parameter in test.
graft/evm/sync/evmstate/firewood_syncer.go	Updates FirewoodSyncer.s type parameter to *ChangeProof.

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