fix(fetchium): Fix declaration emit and rest subpath for downstream consumers

[jimmy-phantom]

Summary

• Fix TS4029 declaration emit errors for downstream packages that compile with declaration: true. Replaced the unique symbol brand on TypeDef<T> with a string literal brand (__fetchiumTypeDef). TypeScript's declaration emitter cannot reference unique symbol across package boundaries (microsoft/TypeScript#56107), causing "cannot be named" errors whenever a combinator like t.optional() is used in an exported class.
• Add rest subpath to Vite build so that fetchium/rest produces runtime JS bundles. The package.json exports declared ./rest but no JS was being built for it.

Changes

1. packages/fetchium/src/types.ts — Replace unique symbol brand with string brand
2. packages/fetchium/src/typeDefs.ts — Cast defineObject assignment to fix Record<string, TypeDef> constraint mismatch caused by string brand
3. packages/fetchium/vite.config.ts — Add rest/index entry point

Verification

• All 1124 unit tests pass
• npm run build succeeds, producing rest/index.js for all output variants
• @phantom/fetchium-client compiles with declaration: true producing zero errors (was 367 TS4029 errors before)
• Wallet repo successfully consumes the built fetchium-client

Trade-off

String brands are slightly weaker than unique symbol (another library could theoretically define a type with __fetchiumTypeDef), but no real-world collisions have been reported for any library using this pattern (Drizzle ORM, ArkType, fp-ts, TypeScript compiler itself all use string brands). The unique symbol approach is fundamentally incompatible with TypeScript's declaration emitter for inferred types across package boundaries.

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