fix(core/emitter): widen string-literal enum tag to u16 for 257+ members

[zuohuadong]

Summary

R5 emits every string-literal union as enum(u8). Once a union has 257 or more members, the zero-based ordinals exceed the u8 range and Zig rejects the generated declaration (enum(u8) cannot hold member value 256). Apps with large generated unions currently need a local overlay to build.

This change uses enum(u16) only when members.length > 256; unions with 256 members still occupy ordinals 0..255 and stay enum(u8). Wire order, payload layout, and the one-line/multi-line formatting paths are unchanged.

Why this is general (not app-specific)

The u8 ceiling is a property of Zig's enum(u8) representation, not of any app's domain. Any Native SDK app whose transpiled Msg/catalog union crosses 257 members hits this today and must carry a local emitter patch. Keeping it local means every SDK upgrade re-baselines the overlay; landing it upstream removes that surface for everyone.

Behavior

members	before	after
3	enum(u8)	enum(u8) (unchanged)
256	enum(u8) (overflows at runtime once a member maps to 255 is fine, but a 257th member cannot be represented)	enum(u8) (unchanged; 256 members occupy 0..255)
257+	enum(u8) (fails to compile)	enum(u16)

Test coverage

• Existing R5 string-literal union becomes a wire-stable enum still asserts enum(u8) for 3 members.
• New R5 string-literal union with 257+ members emits enum(u16) asserts enum(u16), m0 = 0,, m256 = 256,.
• New R5 string-literal union with exactly 256 members stays enum(u8) guards the boundary.

`packages/core` test suite: 152/152 pass with the change.

Compatibility

No wire/protocol change: the on-the-wire tag is the declaration ordinal, and consumers that compare by tag name (.all, .done, switch scrutinee) are unaffected. A u16 tag is a strict superset of u8 for apps that currently fit in u8.

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