customanchor: preview output commitments

[darioAnongba]

Summary

• expose a read-only output commitment preview on CustomAnchorPlan
• derive exact Taproot Asset and Taproot merkle roots without signing, committing, or mutating the plan
• cover caller-witness and backend-signer plans, split outputs, deterministic repeated previews, and index-sensitive rebuilds
• assert preview/sealed-root parity in the existing real-tapd script-path integration test

Motivation

Hosts that embed Taproot Asset outputs in a larger transaction need the final Taproot Asset pkscripts before they can canonically order equal-value outputs. Split commitments bind the anchor output index, so the host must be able to rebuild and preview until the output permutation stabilizes before committing.

Testing

• make fmt
• make unit
• go test -race . -run TestCustomAnchorOutputCommitmentPreview -count=1
• go test -tags=itest ./itest -run ^$
• GOWORK=off go run -modfile=tools/go.mod github.com/golangci/golangci-lint/v2/cmd/golangci-lint run --concurrency=4

Closes #165.