fix(spine): nil-check remoteFeature in SubscribeToRemote/BindToRemote (minimal patch for #87)

[terabytegmbh]

Fixes #87. Minimal-scope alternative to #88.

Problem

FeatureLocal.SubscribeToRemote and BindToRemote dereference remoteDevice.FeatureByAddress() without a nil-check. When a remote device advertises a feature address that the local entity model has not yet materialized, FeatureByAddress returns nil and the subsequent .Type() / .Role() call panics inside the spine.(*events).Publish goroutine, killing the process.

Reproduced reliably in eebus-go EG-LPC use case startup: lpc.(*LPC).connected() runs Subscribe() while the remote's LoadControl feature has not yet been published.

panic: runtime error: invalid memory address or nil pointer dereference
spine.(*FeatureLocal).SubscribeToRemote
    spine/feature_local.go:451
eebus-go/features/client.(*Feature).Subscribe
eebus-go/usecases/eg/lpc.(*LPC).connected
    usecases/eg/lpc/events.go:76
created by spine.(*events).Publish

Fix

Mirror the existing nil-check pattern used immediately above (for remoteAddress.Device and remoteDevice): return a proper *model.ErrorType("feature not found") that the calling code already handles as a debug-level non-fatal event.

Scope

• 6 production lines in spine/feature_local.go
• 2 new test cases (TestSubscribeToRemote_NilFeature, TestBindToRemote_NilFeature)
• No behavioural change for the non-nil path

Relation to #88

#88 from @SAY-5 also addresses #87 with the same conceptual approach (nil-check + early error return), but additionally contains a wider refactor (199 changed files, +23573/-4189 LoC). The maintainer may prefer either path:

• This PR: minimal-surface fix, easy to review, focused on the immediate panic
• fix(spine): guard nil remoteFeature in SubscribeToRemote/BindToRemote #88: larger refactor that includes this fix as part of broader changes

Happy to defer to whichever direction you'd like to land — the issue itself is what matters.

Test

go test ./spine/... clean. Two new tests verify the error-returning path is taken when FeatureByAddress returns nil.

Provenance

Patch has been running in our internal synervatt/spine-go fork since 2026-04-21 without regressions in the eebus-go EG-LPC integration path.

— Thomas, Terabyte GmbH