Map Rust u8/u16 to Go uint8/uint16 instead of signed int

[jaideeppyne]

What

The Go generator maps Rust u8 and u16 to signed Go int, even though it maps u32 → uint32 and u64 → uint64.

// core/src/language/go.rs
SpecialRustType::I8
| SpecialRustType::U8      // unsigned, but folded into signed `int`
| SpecialRustType::U16     // unsigned, but folded into signed `int`
| SpecialRustType::I32
| ... => "int".into(),
SpecialRustType::U32 => "uint32".into(),

Why it's a bug

• Internally inconsistent: u32/u64 keep their unsignedness but u8/u16 don't — and Go has native uint8/uint16, so there's no Go-idiom reason to widen them to signed int.
• The repo's own fixture proves the intended contract. use_correct_integer_types feeds u8/u16 and expects type fidelity: Swift emits UInt8/UInt16, Kotlin emits UByte/UShort. Only Go drops the signedness.
• Interop consequence: the generated Go type is signed where the Rust type is unsigned, so a Go service can assign -1 or a value above the type's range into an "unsigned" field; serde on the Rust side then rejects it on deserialize. The correct uint8/uint16 mapping lets Go's own type system enforce what serde already requires.

Currently, right next to each other in use_correct_integer_types/output.go:

E int    `json:"e"`   // Rust u8
F int    `json:"f"`   // Rust u16
G uint32 `json:"g"`   // Rust u32

How

             SpecialRustType::I8
-            | SpecialRustType::U8
-            | SpecialRustType::U16
             | SpecialRustType::I32
             | SpecialRustType::I16
             | SpecialRustType::ISize
             | SpecialRustType::USize => "int".into(),
+            SpecialRustType::U8 => "uint8".into(),
+            SpecialRustType::U16 => "uint16".into(),
             SpecialRustType::U32 => "uint32".into(),

(Kept surgical to the unambiguous unsigned bug; usize → int is a separate, platform-width-dependent question and left out of scope.)

Tests

Regenerated the Go snapshots with UPDATE_EXPECT=1. The diff is only int → uint8/uint16 for u8/u16 fields across 7 fixtures (e.g. Age, Red, OptionalU16 → *uint16, and use_correct_integer_types's E/F); i32 fields such as ExtraSpecialField1 correctly stay int. The snapshot suite is the regression guard — reverting only go.rs (keeping the corrected snapshots) makes the Go snapshot tests fail. Full typeshare-core suite passes (361 tests), cargo fmt --check is clean.

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Disclosure: this change was prepared with AI assistance and reviewed/verified by me before submission.