Sled-agent: detect switch backend (tofino, softnpu, et al) at runtime

[sion42x]

Replace the switch-asic, switch-stub, and switch-softnpu cargo features with startup detection. A Tofino node in the device tree selects the Tofino ASIC; otherwise a virtio 9p device answering 9P2000.P4 selects the propolis SoftNPU backend, and with neither the existing auto Tofino path is used. Optional switch_backend config added for the stub and SoftNPU zone backends. pumpkind is gated on Oxide hardware.

This PR is a proposed solution to #11202 based on feedback from the control plane team. Prior proposed/attempted solutions:

1. Don't detect, just publish the softnpu sled-agent with CI as a downloadable artifact: Publish the omicron-sled-agent package as a separate artifact for dev labs #11133 (half measure and doesn't solve the real issue)
2. Extending SMBIOS Type 1 table to label the device (results in a new propolis API version, overkill)
3. Creation of a named pci device in propolis for softnpu (probably unnecessary, read on)
4. Detection of the existing vio9p device based on a new subsystem ID (breaks driver loading due to illumos device binding and scadm hardcoding, likewise Mode::Modern requires newer stlouis changes and would be the first modern-only device)

Options 2-4 would have also required additional tooling in omicron to detect these things, making it a pretty sizeable cross-repo change. However, @rcgoodfellow proposed a detection method scadm already uses:

• look for p9fs devices using devinfo
• ask the p9 server if it is 9P2000.P4
• profit

So rather than changing propolis, this PR uses that method to detect softnpu devices. A test on a racklette vs on a voxel lab showed:

Rack	Sleds	Detection log	pumpkind	Dendrite
madrid	14, 16	found tofino node	imported, online	/dev/tofino/1
madrid	15, 17	none	imported, online	n/a
voxel	g0, g2	found SoftNPU 9p device	skipped, no instance	switch0 online
voxel	g1	none	skipped, no instance	n/a

Lastly, follow ups:

• sled_mode, sidecar_revision, and switch_backend overlap, and legal combinations are enforced by runtime checks. It would probably be good to get the latter two into one switch enum so illegal configs aren't representable.
• Switch zone contents are still chosen at build time, but dendrite has an issue for that: One build multiple ASICs dendrite#355

[sion42x]

Note for reviewers: I realized detect_switch_hardware should just use get_tofino_snapshot. The latter requires node.has_asic() and they both resolve identically downstream (TofinoView::Real either way), but folding it into one pre-existing surface makes more sense. I'm working on that now.

[dancrossnyc]

This loop kinda-sorta feels like a hand-rolled version of Iterator::find:
https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.find

? NodeWalk implements Iterator, so it may be useful to pull the bulk of logic here out into a predicate function. Dunno.

[sion42x]

Hmm... I can pull out the per-node logic and into a probe_node predicate. The catch with find itself is that each step is fallible and needs ? to propagate. try_find would make a good fit but I believe it's not stable. So I could do something like:

devinfo
    .walk_node()
    .map(|node| probe_node(log, &node.map_err(SwitchDetectError::DevInfo)?))
    .find_map(Result::transpose)
    .transpose()

but that feels harder to read than a loop. Not to mention doing a double transpose just to make it work which feels icky. Let me know which you think is clearer.

[sion42x]

Tests post c6b42fc on Madrid:

[madrid]root@oxz_switch0:~# pilot host exec -c 'grep -h -E "found tofino asic|found SoftNPU 9p device|probe failed|skipping pumpkind" /var/svc/log/oxide-sled-agent:default.log /pool/ext/*/crypt/debug/global/oxide-sled-agent* 2>/dev/null | cut -c1-150' 14-17
14  BRM42220081        ok: {"msg":"found tofino asic","v":0,"name":"SledAgent","level":30,"time":"1986-12-28T00:00:44.501031762Z","hostname":"BRM42220081","pid":673,"component":
15  BRM42220046        ok: 
16  BRM42220007        ok: {"msg":"found tofino asic","v":0,"name":"SledAgent","level":30,"time":"1986-12-28T00:00:43.850669382Z","hostname":"BRM42220007","pid":674,"component":
17  BRM42220004        ok: 

On voxel:

root@helios1:~# for i in g0 g1 g2; do
> voxel host exec -c 'grep -h -E "found tofino asic|found SoftNPU 9p device|probe failed|skipping pumpkind" /var/svc/log/oxide-sled-agent:default.log /pool/ext/*/crypt/debug/global/oxide-sled-agent* 2>/dev/null | cut -c1-150' $i
> done
Sep 05 10:26:40.842 INFO set propolis binary to /root/propolis-9pexp/target/release/propolis-server
{"msg":"not an Oxide sled; skipping pumpkind","v":0,"name":"SledAgent","level":30,"time":"2026-09-04T17:35:22.830647805Z","hostname":"g0","pid":792,"c
{"msg":"found SoftNPU 9p device","v":0,"name":"SledAgent","level":30,"time":"2026-09-04T17:35:23.647422767Z","hostname":"g0","pid":792,"component":"Bo
Sep 05 10:26:59.634 INFO set propolis binary to /root/propolis-9pexp/target/release/propolis-server
{"msg":"not an Oxide sled; skipping pumpkind","v":0,"name":"SledAgent","level":30,"time":"2026-09-04T17:35:08.525600963Z","hostname":"g1","pid":741,"c
Sep 05 10:27:14.049 INFO set propolis binary to /root/propolis-9pexp/target/release/propolis-server
{"msg":"not an Oxide sled; skipping pumpkind","v":0,"name":"SledAgent","level":30,"time":"2026-09-04T17:36:12.543212179Z","hostname":"g2","pid":851,"c
{"msg":"found SoftNPU 9p device","v":0,"name":"SledAgent","level":30,"time":"2026-09-04T17:36:13.375344975Z","hostname":"g2","pid":851,"component":"Bo

Working as expected on all scrimlet/non-scrimlet + rack/voxel combinations.