Add `Run#getWritable()` for appending to another run's stream (#3972)

## Summary & Motivation

Lets a long-lived run own a shared stream while independent runs append through `run.writable` or `run.getWritable()` using only the owner's run ID. The handle seals to the owner's public key when it has one, so it grants append access without read capability, and carries the existing forwarding symbols so passing it through `start()` and into a step keeps the owner's identity.

## Test Plan

Tests added