RFC: Host locale subscription

[kalininilya]

RFC + protocol implementation for a host locale subscription.

locale.subscribe() pushes the language the host presents its interface in, as a BCP 47 tag, on subscribe and on every change. A product renders in that language instead of inferring one from navigator.language, which reports the operating system's preference and is wrong whenever the user picked a different language inside the host.

The RFC and the crate change travel together because check-rfc.yml fails any PR touching docs/rfcs/** that does not also change rust/crates/truapi/.

Shape

struct HostLocaleSubscribeItem {
    /// BCP 47 language tag.
    pub language_tag: String,
}

Wire ids 194–197. LocaleHost joins the required Platform set alongside ThemeHost, so a host serves a real value rather than reporting the method unsupported.

The tag is an open string rather than an enum of shipped languages: a host adds a language without a protocol change, and a product that ships no catalog entry for the tag it receives picks its own fallback.

Verified

• cargo test --workspace — 1052 passed, 0 failed
• cargo +nightly fmt --check and cargo clippy --workspace --all-targets --all-features -- -D warnings clean
• ./scripts/codegen.sh regenerated the client, dispatcher, wire table and WASM bridge; golden diffs are locale-only
• js/packages/truapi: npm run build and npm test — 255 passed

Tracking issue follows.