v0.7.4

0.7.4 — forward-compatible capability verification

Clients verify a signed directory over the bytes as signed and tolerate capability fields they do not recognize, instead of rejecting the directory. A future caps.* field no longer bricks an installed client the way caps.sets did to v0.7.0; interpretation still ignores unknown fields. Symmetric in the Rust client and the JS node, with a previous-release conformance vector. No new signed capability field ships until this release is the floor.