pcool: Client-side end-to-end latency measurement

> **Epic** — break into sub-tasks when work begins.

Server-side latency measurements (#10750) are not observable by clients. Implement client-side latency tracking in the SDK so clients can independently select the best fullnode.

Depends on: #10750

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### Suggested sub-tasks (create when starting)
- Design per-fullnode score structure and decay algorithm (mirror #10750 model)
- Implement submission-to-effects latency recording in Rust SDK
- Implement weighted fullnode selection based on recorded scores
- TypeScript SDK equivalent (if coverage needed)
- Tests and benchmarks

This is an epic with no files or tests named; start by reading issue #10750 and identifying the Rust SDK entry points for submission-to-effects latency and fullnode selection. Break the work into the listed score, recording, selection, TypeScript, and test/benchmark sub-tasks, with completion defined by client-side measurements and weighted selection covered by tests.

Written by the indexing model from the issue text.