Polish partial load command handling

[chboishabba]

This follow-up cherry-picks the second commit from PR #26 as a separate change set.

What it adds

• JSON string escaping for .index-file output.
• Overflow-safe chunk index parsing.
• REPL state cleanup when .new resets the network.
• A source-bin fallback in manifest mode when the manifest path itself ends in .bin.

Validation

• Cherry-picked cleanly from 84844b7.
• Local rebuild passed.