Add targeted dependency updates

[LunaStev]

Summary

• add vex update <package>... for targeted Git dependency updates
• preserve unrelated locked commits and avoid fetching unrelated repositories
• support selecting transitive packages and recalculating changed subgraphs
• document the command and add local-Git integration coverage

Why

The existing resolver exposed only a boolean update mode, so vex update always refreshed every Git dependency. Projects need to advance one dependency without changing unrelated locked SHAs or contacting unrelated repositories.

Behavior

• vex update continues to refresh the complete Git dependency graph.
• vex update alpha [beta...] refreshes only the named Git packages.
• Selected transitive packages are supported.
• New manifests from selected packages can add, remove, or change downstream graph entries while existing cycle, source, version, and package-name checks remain in effect.
• Unknown names report the available Git packages and an actionable command.
• --locked and --offline remain rejected for update.
• Help and README usage now show vex update [<package>...].

Validation

• cargo fmt --check
• cargo test --locked
• cargo clippy --locked --all-targets -- -D warnings
• cargo build --locked
• local-Git integration tests for targeted SHA preservation, no unrelated fetch, full update compatibility, transitive selection, graph recalculation, unknown packages, and update policy flag rejection
• smoke: PATH-resolved wavec ran Hello World; raw --emit=obj stayed rejected; fetch/build --locked --offline succeeded