refactor(hm-plugin-protocol): Mark the wire-protocol BuildEvent enum #[non_exhaustive]

[markovejnovic]

The smell

BuildEvent in crates/hm-plugin-protocol/src/events.rs is the central wire event type for the build event stream. It is a plain, exhaustive pub enum, matched exhaustively in hm-render (progress.rs, human.rs) and hm-exec. The protocol is explicitly designed to grow — several variants' doc comments already describe them as "replacing ad-hoc log lines," and more event kinds are expected over time.

Because the enum is exhaustive, adding any new variant is a breaking change for every downstream match and a semver-major bump for the crate.

The change

Add #[non_exhaustive] to enum BuildEvent. This is a single-attribute change in this crate. Downstream match arms over BuildEvent will then need a _ => {} wildcard, after which new variants can be added without breaking external callers.

Type-safety pattern

This applies the #[non_exhaustive] pattern used by diesel and ripgrep (and the standard library's own error/event enums) for public enums that are expected to grow — error kinds, event types, message kinds. It converts "adding one variant is a major break for every matcher" into a non-breaking change, while the compiler still enforces exhaustiveness for matches inside this crate. BuildEvent is the prototypical growing event stream.

Validation

Only cargo check -p hm-plugin-protocol was run locally (passes). The full build and TS build were intentionally not run. Full CI runs on this PR.

This is a draft pending review. The change is behavior-preserving (an attribute only); it does not alter serialization or runtime behavior. Downstream crates in this same workspace already match exhaustively, so CI will confirm whether any in-workspace matcher needs a wildcard arm added (a non-#[non_exhaustive] enum allows in-crate exhaustive matches, but #[non_exhaustive] only forces wildcards for out-of-crate matchers — so in-workspace matches should be unaffected).