feat(v3): add GraftConfigTreeNode to materialize anonymous config tree nodes

[ScreamingHawk]

What

Adds v3.GraftConfigTreeNode(tree, replacement) — swaps the first anonymous node-hash leaf (WalletConfigTreeNodeLeaf) whose image hash equals replacement.ImageHash() for the full replacement subtree. The tree's image hash is unchanged by construction: a swap only happens on hash equality. Idempotent: if the hash is already materialized as a full subtree, the tree is returned unchanged with ok=true.

Why

BuildIntentConfigurationSignature / BuildRegularSignatureFromSignatures can only attach a supplied signer signature to a full leaf. Configs decoded from keymachine can carry sapient leaves (e.g. the Trails timed-refund leaf) as anonymous node hashes — no address — so the supplied sapient signature silently contributes zero weight and the on-chain threshold fails.

Both trails-api and trails-watchtower currently carry private copies of this graft (graftSapientLeaf). This upstreams it generically: any subtree can be grafted, and hash-equality matching makes the substitution provably config-preserving without a separate verification step. Related: the commented-out reverse-direction replaceSapientSignerWithNodeInConfigTree in intent_config.go.

Tests

core/v3/graft_test.go: value and pointer node leaves under WalletConfigTreeNode, node leaf under WalletConfigTreeNestedLeaf (Weight/Threshold preserved), already-materialized subtree, hash absent, nil inputs. Every graft asserts image-hash equality with the original tree and that the input tree is not mutated. Includes a non-sapient replacement to cover the generic case.