fix(dist): verify release artifact builds

[Lenvanderhof]

Summary

• Add the cargo-dist release profile required by actual artifact builds.
• Add a real host archive/installer/checksum build to local and hosted CI.
• Add a regression contract that fails if the profile or build gate disappears.

Root cause

The existing manifest-only validation accepted the configuration without invoking Cargo. The first real dist build then failed because profile dist was undefined.

Verification

• Reproduced the failure with dist build --artifacts=host --tag v0.2.0.
• Added a test and observed the expected missing-profile failure.
• Re-ran the test successfully after the minimal fix.
• just ci passes: 104 Rust tests, 35 Python tests, protocol matrix, adversarial gates, audit, package, Registry schema, and real cargo-dist host artifacts.