Replace runtime String errors with a typed L10nError

[akesson]

Third of the batched 0.7 breaking changes on the road to 1.0 (road-to-1.0 item #3; version already at 0.7.0).

What & why

The runtime API returned Result<_, String> everywhere — impossible to match on, and a 1.0 wart. This introduces L10nError, a #[non_exhaustive] enum re-exported from the crate root and the prelude, and switches the whole runtime surface to it:

• L10nBundle::{new, new_without_isolation, msg, attr, msg_segments}
• L10nLanguageVec::{load, load_without_isolation}
• the generated L10nLanguage::new
• the generated compressed L10n::{load, load_all} accessors — the user's decompressor still returns Result<_, String>, and its error is wrapped in L10nError::Decompression

It mirrors the BuildError work already done on the build side. Crucially, the variants carry owned strings, not fluent-bundle's error types, so this public enum doesn't churn when that pre-1.0 dependency is bumped — the same decoupling the road-to-1.0 plan calls for.

This is breaking only for code that matched on / compared the error String. Code that unwrap/?-ed is unaffected — which is why the generated accessors (all .unwrap()) needed no body changes.

Notable mechanics

• The committed *_gen.rs test modules are compiled, and their L10nLanguage::new does L10nBundle::new(...)? — so once new returns L10nError, the stale String versions wouldn't compile (no String: From<L10nError>), which would block the very test run that regenerates them. I pre-applied the exact signature change, then let the tests regenerate and confirmed the only content diff is the new() line.
• Refreshed all insta snapshots (cargo insta accept); verified the aggregate diff across 15 snapshots is only the signature line.
• Regenerated the playground outputs (incl. the compressed single_gzip_l10n.rs).
• Strengthened the runtime test to assert L10nError::MessageNotFound { id } instead of just .is_err().

Verification (local, all green)

• cargo test --features build,langneg — 100 unit + 1 playground integration test
• cargo fmt --all --check, cargo clippy --all-targets --features build,langneg -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --features build,langneg
• cargo build --benches --features bench-internals

Scope note

impl FromStr for L10n still uses type Err = String (a generated enum-parse on a generated type, distinct from the runtime error path) — left as-is to keep this focused.

Remaining 0.7 work

4. Remove the public-API panics (L10nLanguageVec::get, generated load()/load_all() unwraps).

🤖 Generated with Claude Code