Compat baseline 3/5: make minHost metadata actionable (diagnostics, prerelease-aware comparison) #133

Effort: M

Part of #102. Depends on Compat baseline 1/5.

minHost is exposed by the plugin loader as metadata only; no frontend enforcement exists, and existing manifests carry placeholder values (e.g. 1.0.0) that match no real dependency. Make the field useful without enforcing wrong values.

Scope

• Implement a prerelease-aware version comparison (the host is on 0.3.0-alpha.N; plain semver comparison mishandles prereleases).
• Surface a clear diagnostic when a plugin's declared minimum is not met (what plugin, what it needs, what the host is), instead of silently loading or silently skipping.
• Do not blindly enforce existing placeholder values such as 1.0.0: define how to treat unset, placeholder and real values, and document the rule.
• Make the diagnostic available in the Settings diagnostics bundle.

Acceptance criteria

• A plugin declaring a real minimum above the host version gets a diagnostic naming the gap.
• Prerelease ordering is correct (0.3.0-alpha.2 < 0.3.0-alpha.10 < 0.3.0).
• Plugins with unset or placeholder minHost behave exactly as today.
• Unit tests cover the comparison and the three value classes.

Out of scope

Choosing the baseline version (sub-issue 5).