feat: add RootHashPadded for fixed-height subtree composition

[ordishs]

Summary

Adds Subtree.RootHashPadded(targetHeight int) (*chainhash.Hash, error) — a method that computes the subtree's merkle root and then lifts it to a caller-specified height by repeatedly hashing the root with itself (H(prev, prev) per phantom level). This is bitcoin's "duplicate-last-when-odd" rule applied to phantom slots above the actual leaves.

The motivation is to make subtree-root composition produce the same merkle root as building a single big merkle tree over all leaves, even when the last subtree is partial. Today, combining a complete subtree's root with a partial subtree's un-padded root yields a different value than the bitcoin merkle root over the same transactions — RootHashPadded closes that gap. Consumer (Teranode) intends to use it for block validation where one block carries multiple fixed-height subtrees and the final one may be incomplete.

What's in the PR

• errors.go: two new sentinels — ErrTargetHeightTooSmall, ErrNotPowerOfTwoLeafCount.
• subtree.go: the RootHashPadded method, placed adjacent to RootHash.
• subtree_padded_test.go: seven tests covering the happy path, no-lift, empty-subtree, both error paths, an 8+2-leaves big-tree equivalence check, and a block-scale 258-tx (256+2 composition) equivalence check.

Purely additive — no existing behaviour changes.

Preconditions enforced by RootHashPadded

• len(Nodes) must be a power of two (or zero) → otherwise ErrNotPowerOfTwoLeafCount.
• targetHeight >= ceil(log2(len(Nodes))) → otherwise ErrTargetHeightTooSmall.
• Empty subtree: returns (nil, nil) (matching RootHash's nil-for-empty contract; guarded by a //nolint:nilnil directive with explanation).

Test plan

• go test -race -count=1 ./... — 76 tests pass, 0 failures
• go vet ./... — clean
• golangci-lint run --new-from-rev=master — 0 issues
• go build ./... — clean

Note on archive status

I see the master README now carries an archive notice pointing to the go-stack monorepo. Happy to redirect this to go-stack if maintainers prefer — let me know. Submitting here first since (a) the repo isn't actually archived yet and (b) downstream consumers' go.mod still points here.

[github-actions]

👋 Thanks, @ordishs!

This pull request comes from a fork. For security, our CI runs in a restricted mode.
A maintainer will triage this shortly and run any additional checks as needed.

• 🏷️ Labeled: fork-pr, requires-manual-review
• 👀 We'll review and follow up here if anything else is needed.

Thanks for contributing to bsv-blockchain/go-subtree! 🚀

[mrz1836]

LGTM