v0.6.4

Patch release: fix patch_source tripping the host loop supervisor.

Successive surgical edits returned a byte-identical success envelope, so a normal batch-patch workflow (6–10 edits) tripped the supervisor's repeat-result guard after the 3rd edit and lost write access mid-document. WorkspaceState now carries a per-call source_sha (sha256 of the current source), so successive edits are distinct. Also fixes the latent identical issue in set_source / set_theme.

See #12.