feat(runtime): host GuestExec layer + portable exit-signal error type (RIG-2493)

[rigel-mintaka]

This PR is part of a stack containing 5 PRs:

1. main
2. feat(guest_control): grow GuestControl with the V2b exec surface (RIG-2493) #696
3. feat(guestd): replace the Health-only stub with the V2b in-guest supervisor (RIG-2493) #701
4. "feat(runtime): host GuestExec layer + portable exit-signal error type (RIG-2493)" (this PR)
5. feat(runtime): fill MicroVMRuntime lifecycle methods (RIG-2493) #719
6. test(runtime): shared ContainerRuntime contract suite across both backends (RIG-2493) #720

U3 + U3b of the frozen microVM Runner V2b plan (docs/designs/infra/runtime/compass-elastic-session-runtime/microvm-v2b-guest-supervisor-exec.md, §Plan U3/U3b, §(c)/§(e)). The host-side client half of the exec control plane: a GuestExec layer over U1's generated GuestControl client, plus the backend-portable error type the runner's kill-classifier keys on.

U3b — portable exit-signal error (internal/runtime/exit_status_error.go, new)

ExitStatusError{Code int; Signal syscall.Signal} with Error(), in package runtime. A remote guest exec cannot forge an *exec.ExitError, so the microVM waitFunc constructs this instead for a signalled guest exit. internal/runner/agent_exec.go's isDeliberateKill is widened to errors.As on *runtime.ExitStatusError first (a non-zero signal ⇒ deliberate kill), then the existing *exec.ExitError SIGKILL branch unchanged — so the podman byte-path is byte-identical and only the microVM backend gets the new classification.

U3 — GuestExec layer (internal/runtime/microvm/exec.go, new)

Plain structs (ExecCall/ExecResult/StreamCall/ExitStatus) mirror the proto so runtime types don't leak into microvm (microvm must not import runtime; U4 makes runtime import microvm).

• Exec (one-shot RPC): a non-zero exit is a successful ExecResult; a refusal/transport failure is an error; a host-side timeout → *microvm.TimeoutError.
• ExecStream (bidi): opens the stream, sends StartExec, awaits ExecStarted before returning; per-exec goroutines pump the Stdin pipe → stdin frames (Close → stdin_close + CloseRequest) and demux stdout/stderr response frames onto io.Pipe read ends, closing them on ExecExit.
• GuestStream.Kill issues a Signal RPC bounded by killSignalTimeout (never blocks teardown past it); Wait blocks on the demux goroutine and returns ExitStatus{Code,Signal}. A ctx-cancel stream break reports SIGKILL — guestd reaps the child bound to the broken stream.

podman.go — funcs-backed ChildHandle

Adds unexported newChildHandleFuncs(kill, wait func() error) *ChildHandle plus killFunc/waitFunc fields. Kill/Wait/Terminate branch to the funcs path when killFunc != nil, else drive the original cmd/cancel path untouched (podman byte-unchanged). Terminate = kill then wait, returning the wait error (the exit status) when non-nil else the kill error — so a crash-vs-teardown consumer gets the exit signal.

Verification

go build ./internal/runtime/... ./internal/runner/... clean; go vet clean over runtime, runtime/microvm, runner. Tests (all hardware-independent, no microvm build tag, no KVM):

• exec_test.go — fake GuestControl server on a unix listener over h2c Connect, dialed by a real client: one-shot non-zero exit is success + spec mapping; refusal is an error; one-shot timeout → *TimeoutError; stream demux ordering (interleaved frames land on the correct pipes, drained concurrently); stdin framing + Close → stdin_close; Kill issues the Signal RPC and Wait unblocks on the signalled exit; ctx-cancel breaks the stream and Wait reports SIGKILL.
• child_handle_test.go — newChildHandleFuncs: Kill invokes/propagates; Wait surfaces *ExitStatusError unchanged; Terminate return-value precedence.
• deliberate_kill_test.go — isDeliberateKill accepts a real *exec.ExitError from a SIGKILLed local child (podman-path regression guard) and *runtime.ExitStatusError{Signal: SIGKILL}, rejects a non-signal ExitStatusError, an unrelated error, and nil.

Ledger-impact: none

Spec-impact: none. Refs RIG-2493

Co-authored-by: Matt Wilkinson matt@rigel.build

[linear-code]

RIG-2493

[github-actions]

Compass engineering docs preview: https://compass-runner-rig-2493-u3-h.compass-eng-docs.pages.dev

Deployed from compass-runner/rig-2493-u3-host-exec at 417292b.