refactor: use boxed watcher, not watchable, on connection

[Frando]

Description

Alternative to #3631

Replaces the Watchables for path changes on the Connection with a boxed Watcher. The watcher is boxed because it would increase the Connection struct size significantly otherwise because the mapped-and-joined watcher with a SmallVec of PathInfo inside is ~600 bytes atm.

The benefit of storing a Watcher and not a Watchable is that the watcher streams now close once the EndpointStateActor drops the state for the connection, which it does after the connection is closed.

Also adds a test for path watching, including testing that the streams now close when the connection closes.

Breaking Changes

Notes & open questions

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented.
  • List all breaking changes in the above "Breaking Changes" section.
  • Open an issue or PR on any number0 repos that are affected by this breaking change. Give guidance on how the updates should be handled or do the actual updates themselves. The major ones are:
    • quic-rpc
    • iroh-gossip
    • iroh-blobs
    • dumbpipe
    • sendme

[github-actions]

Documentation for this PR has been generated and is available at: https://n0-computer.github.io/iroh/pr/3632/docs/iroh/

Last updated: 2025-11-11T16:00:29Z

[matheus23]

LGTM. Personal preference would be to switch the inner Box from being referenced as .0 to.inner, but that's a minor nit.

[Frando]

I'd be in favor of merging this, it is the cleaner design for now. I don't like storing the Watchable in the Connection because this keeps the path watcher alive after the connection closed if there is still a strong clone of Connection somewhere.

[github-actions]

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Last updated for commit: 02c09e2

[flub]

Yeah, LGTM. I was just wondering if that ARC was possible.