fix(types): SorokitClient submit accepts unknown, InvokeParams args too permissive, ContractEvent value typed as unknown

[k-deejah]

Problem

Three issues affect the developer experience when using the library in a TypeScript strict-mode project.

1. SorokitClient interface submit(tx: unknown) accepts any value — loses type safety

transaction.submit accepts tx: unknown which means TypeScript cannot catch incorrect payload shapes at the call site. A typed TransactionParams interface replacing unknown would catch errors at compile time.

2. InvokeParams.args typed as unknown[] — callers cannot express Soroban XDR types

args?: unknown[] is too permissive. Callers passing scVal objects from @stellar/stellar-sdk have no type-safe way to declare them. A ScVal[] or a union type would enable type checking at the Soroban call site.

3. ContractEvent.value typed as unknown — JSON.stringify result cannot be typed

event.value: unknown means JSON.stringify(event.value, null, 2) in EventRow works at runtime but has no type-level structure. A branded JsonValue type or Record<string, unknown> would at least constrain the shape and enable smarter rendering decisions.

Solution

1. Define TransactionParams interface in client.ts and replace submit(tx: unknown) with submit(tx: TransactionParams).
2. Change InvokeParams.args from unknown[] to SorobanScVal[] | unknown[] with JSDoc explaining the Soroban XDR types.
3. Change ContractEvent.value from unknown to JsonValue (a recursive type representing JSON-serialisable values).

Acceptance Criteria

• TransactionParams interface defined and used in SorokitClient
• InvokeParams.args JSDoc explains Soroban XDR types
• ContractEvent.value typed as JsonValue
• tsc --noEmit passes with no new errors
• npm run build passes

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Note for Contributors: Write a clear PR description. Show the TypeScript errors that disappear after the TransactionParams change and confirm the JsonValue type covers all serialisable JSON shapes.