fix(runner): guard microVM launcher Pdeathsig behind a linux build tag (RIG-3213) - #848

…g (RIG-3213)

`launch.go` is tagged `//go:build unix` (which includes darwin) yet named
`syscall.SysProcAttr{Pdeathsig: syscall.SIGTERM}` directly. `Pdeathsig`
(PR_SET_PDEATHSIG) is a Linux-only field, so any darwin compile of the
microvm package failed:

    launch.go:298:43: unknown field Pdeathsig in struct literal of type
    syscall.SysProcAttr

Latent until now — main's darwin gate does not compile microvm — but
restructured build tags pull microvm into the darwin compile of
`./cmd/compass-app`, exposing it, and it traps any future PR that
compiles microvm on darwin.

Extract the SysProcAttr construction into a platform helper:

- `orphanguard_linux.go` (`//go:build linux`) sets Pdeathsig (behavior
  byte-identical to today).
- `orphanguard_other.go` (`//go:build unix && !linux`) returns nil — no
  PR_SET_PDEATHSIG equivalent on darwin. The launcher only runs for real
  on Linux/KVM and teardown correctness rests on Shutdown, not the guard
  (record §(g)), so the darwin no-op loses nothing that matters.

`launch.go` calls the helper instead of naming the Linux-only field, so
the unix-tagged package cross-compiles on darwin.

Verified: `GOOS=darwin GOARCH=arm64 go build ./internal/runtime/microvm/`
was red, now green; linux build (plain + `-tags microvm`) green; untagged
microvm unit tests pass; full lint 0 issues; nilaway clean.

Refs RIG-3213, RIG-2591
Co-authored-by: Matt Wilkinson <matt@rigel.build>

…213)

Rename the two orphan-guard platform files so their build constraint lives
solely in the explicit //go:build tag, not split across a tag and a GOOS
filename suffix:

- orphanguard_linux.go   -> orphanguard_pdeathsig.go  (//go:build linux)
- orphanguard_other.go   -> orphanguard_noop.go       (//go:build unix && !linux)

`_linux.go` carried an implicit `//go:build linux` constraint from its GOOS
suffix on top of the explicit tag; the companion `_other.go` relied on the
explicit tag alone (no single GOOS suffix expresses "all non-linux unix").
Using one mechanism for both — explicit tags, descriptive names — is the
consistency the split was meant to express. Content is unchanged; the
partition is identical (linux+android -> pdeathsig helper; other unix ->
noop helper).

Refs RIG-3213
Co-authored-by: Matt Wilkinson <matt@rigel.build>