docs: align Buffer wording with Uint8Array-first v7 APIs

[lindsey-comet]

Summary

Updated README.md and JSDoc comments to reflect that v7 APIs are Uint8Array-first. The documentation now consistently describes Uint8Array as the canonical public type while keeping explicit compatibility notes where Buffer input is accepted.

Changes

• README.md: Changed 'Node Buffer'''s throughout' to 'Uint8Array (with node Buffer supported as a Uint8Array subclass) throughout'
• ts_src/address.ts: Updated JSDoc to show Uint8Array as primary type with Buffer compatibility note
• ts_src/payments/bip341.ts: Updated 2 JSDoc @returns comments
• ts_src/script.ts: Updated JSDoc to reference Uint8Array instead of Buffer
• ts_src/psbt/psbtutils.ts: Updated JSDoc @returns comment
• ts_src/psbt/bip371.ts: Updated JSDoc @returns comment
• ts_src/types.ts: Updated JSDoc @param comments

Acceptance

• Docs consistently describe Uint8Array as canonical public type
• Compatibility statement remains explicit for existing Buffer-accepting paths
• No runtime behavior changes
• No API signature changes

Fixes #2321

[jvgelder]

duplicate of #2322 ?