Upstream tracking: migrate BlueyeApp-specific patches to zeromq/netmq

[follesoe]

Goal

Stop maintaining a long-lived divergent fork. Migrate each BlueyeApp-specific patch back upstream so we can eventually point the BlueyeApp submodule at a fresh fork (or directly at `zeromq/netmq`).

Workflow

1. Create a fresh fork of `zeromq/netmq`.
2. For each tracking issue below, do the upstreaming work listed in the issue: build a deterministic test, prove the underlying bug, open a PR upstream.
3. As PRs land, drop the corresponding patch from the BlueyeApp fork.
4. Once all patches are upstreamed (or proven obsolete), retire the fork.

Current divergence

As of 2026-05-04, the BluEye-Robotics/netmq `dev` branch is 5 commits ahead of `master` (which mirrors upstream/master). 4 of those 5 are upstream cherry-picks already merged at zeromq/netmq; only 1 (`6f73a01a` — `System.Security.Cryptography.Xml` override) is BlueyeApp-only build config that doesn't need upstreaming.

The patches that do need upstreaming all live further back in fork history (2024-05 to 2026-03). They are tracked one-per-issue:

• Upstream: Remove macOS Socket.Select workaround in Poller.cs #2 — Remove macOS Socket.Select workaround in Poller.cs (likely redundant after upstream Add macOS CI job, fix Cleanup hang on macOS, and add regression test zeromq/netmq#1148; verify and drop)
• Upstream: Guard against writing to disconnected Signaler socket #3 — Guard against writing to disconnected Signaler socket (iOS resume hardening)
• Upstream: Ignore CommandType.Done with null Destination in SocketBase #4 — Ignore CommandType.Done with null Destination in SocketBase (from `tmatthey/pr/check-destination-null`)
• Upstream: Defensive exception handling in YPipe.TryRead and Mailbox #5 — Defensive exception handling in YPipe.TryRead and Mailbox (three-commit group from @tmatthey)
• Upstream: Mitigate FaultException in Mechanism.Encode (SessionBase) #6 — Mitigate FaultException in Mechanism.Encode (from `sonicpro/master`, has Sentry signal)
• Upstream: Remove Asumes.NotNull on SizeReady in V2Encoder #7 — Remove Asumes.NotNull on SizeReady in V2Encoder (Debug-only, lowest priority)

Suggested upstreaming order (highest leverage first)

1. Upstream: Defensive exception handling in YPipe.TryRead and Mailbox #5 (Mechanism.Encode FaultException) — has direct production evidence (BLUEYE-APP-FT), strongest case to make to upstream.
2. Upstream: Guard against writing to disconnected Signaler socket #3 (Signaler write guard) — addresses iOS-resume scenario; useful for any mobile NetMQ user.
3. Upstream: Ignore CommandType.Done with null Destination in SocketBase #4 (Done command null Destination) — small, isolated, has external co-author already.
4. Upstream: Defensive exception handling in YPipe.TryRead and Mailbox #5 (YPipe / Mailbox defensive catches) — needs careful test work to prove the underlying race.
5. Upstream: Remove macOS Socket.Select workaround in Poller.cs #2 (macOS Socket.Select removal) — verify obsolescence first; may be a no-op.
6. Upstream: Remove Asumes.NotNull on SizeReady in V2Encoder #7 (V2Encoder Asumes) — Debug-only, defer.

Notes

• Each tracking issue lists the originating commit, the file(s) touched, the original author / external branch where applicable, and the upstream-PR work needed (e.g. tests to write, repros to construct).
• Don't drop any patch from `BluEye-Robotics/netmq/dev` until either the upstream PR has merged AND been released in a NuGet upstream (so the BlueyeApp can pin to that version), or until proven obsolete. Premature drops re-introduce the original bug.