fix(builder): encode ScVals with ABI-correct integer types

[p4uld4vid016-code]

Closes #497

Problem

paramToScVal() in src/batch-tx.ts forced every integer number to i64 and every bigint to i128. create_stream's start_time/end_time are u64 and stream IDs are u64, so passing these through the params path produced the wrong ScVal type and a contract-side type error. The map path had no way to carry per-field type information.

Changes

• paramToScVal(value, type?) — no longer blanket-encodes integers:
  • Untyped positive integers encode as u64, negatives as i64 (the natural SDK encoding, which matches the u64-heavy ABI).
  • An explicit type hint ('u64', 'i128', ...) forces a specific width.
  • Already-encoded xdr.ScVals pass through untouched.
• Map path per-field types — BatchOperation.types (and BuildableOperation.types) supplies per-field ScVal type hints for params map entries, e.g. { method: 'withdraw', params: { streamId: 1n }, types: { streamId: 'u64' } }. ScValType is exported from the package entry point.
• ConduitBatcher.execute() create_stream path — now builds ABI-exact positional args: deposit_amount/rate_per_sec as i128, start_time/end_time as u64 (honoring startTime/endTime/clawbackEnabled when present), clawback_enabled as bool.
• Drive-by: removed a duplicate let consecutiveFailures declaration in src/events.ts (introduced on main in fix(sdk): dedupe progress calc, unify NonceManager, harden event polling (#482, #483, #484, #485) #499) that made the module a SyntaxError, breaking npm run typecheck, the npm ci build, and every event-subscription test; fixed the stale startLedger assertion in events-subscribe.test.ts that could never run before.

Tests

• New src/tests/builder-param-encoding.test.ts covering the ABI-exact execute() create_stream args (decoded from real XDR), per-field types for u64 stream IDs, bigint amounts staying i128, and pre-encoded ScVal passthrough.
• Updated batch-real-xdr.test.ts parameter-encoding tests for the new defaults + type hints + ScVal passthrough.
• npm run typecheck, npm run lint, npm run build, and the full vitest suite (853 passed, 2 skipped) are green.

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