[P3][lens] Handle lone UTF-16 surrogate string keys in kv/doc ids

[cevheri]

Summary

String keys containing lone UTF-16 surrogates encode to UTF-8 replacement or collide after utf8.encode in kv/doc id paths — distinct strings can map to same bytes.

Audit ID: Section 4 Low — Wave 3
Location: src/lens/kv.ts:62 and shared utf8 helpers

Options

1. Reject invalid Unicode in string keys/ids at API boundary (throw).
2. Document that keys must be valid UTF-8 strings; surrogate pairs only.

Acceptance criteria

• Documented or rejected with test vectors.
• Changeset if throwing new errors.

[cevheri]

Implemented and shipped in PR #43 (merge commit 69f7161), released as v0.2.0. The PR description's comma-separated Closes list only auto-closed the first reference — closing manually. For #39/#28-style partials: the shipped scope is done; the real-browser verification half of #39 continues under #10, and the typed-error contract (#28) shipped as LibreDbError with stable codes across kernel, adapters, lenses, and CLI.