test(runtime): shared ContainerRuntime contract suite across both backends (RIG-2493) - #720

…kends (RIG-2493)

U5 of the frozen microVM Runner V2b plan (`docs/designs/infra/runtime/compass-elastic-session-runtime/microvm-v2b-guest-supervisor-exec.md`, §Plan U5) — the V2b acceptance gate. One table-driven contract suite proves `MicroVMRuntime` and `PodmanCLI` behave identically through the `runtime.ContainerRuntime` interface, run against BOTH backends, with the 6 conceded divergences (record 580-593) encoded as a per-backend capability matrix so a divergence that silently *widens* fails a row.

### What

- **`contract_suite_test.go`** (UNTAGGED, package `runtime`): the shared body. `backendCaps` capability descriptor (factory + the 6 divergence flags + typed-error closures) and `runContractSuite`, a thin dispatcher whose rows are one helper each. It references ONLY untagged production symbols plus `backendCaps` — every KVM/podman-only symbol (`microvmtest`, `podmanUsable`, `buildImage`, `*DuplicateNameError`) is reached through a caps closure, so the file compiles on every platform. A `var _ = runContractSuite` roots the graph for the untagged `unused` lint pass (both real callers are tag-gated).
- **`contract_podman_test.go`** (`//go:build podman`): `TestContractSuite_Podman` — skips unless `podmanUsable()`, builds the agent image once, containers run `sleep infinity` as uid 1000; all microVM divergence flags OFF so those rows self-skip; the deliberate-kill row asserts the byte-identical `*exec.ExitError` path (podman byte-path unregressed).
- **`contract_microvm_test.go`** (`//go:build microvm && unix`): `TestContractSuite_MicroVM` — `microvmtest.Require(t)`, sessions with a `/workspace` share + uid 1000; all 6 divergence flags ON so every divergence row runs and a silent widening fails. Plus `TestMicroVMQBudget`, informational boot-latency + per-process PSS via `t.Logf` (record §(g)), NOT a boot gate.
- **Fold, not duplicate**: `TestMicroVMLifecycleEndToEnd` (Create→Start→Exec→Stop→Remove) and `TestMicroVMExecStreamingKillSignalsExit` (ExecStreaming-kill), previously standalone in `microvm_lifecycle_microvm_test.go`, are now shared contract rows and were folded out of that file. `e2eConfig` and `TestMicroVMStartFailureLeavesNoState` (a microVM-backend-only negative with no podman analog) stay.

### Divergence 6 (deliberate-kill error shape)

The shared row body proves the SAME behavioral contract — a deliberate Kill+Wait surfaces as a signalled exit `isDeliberateKill` accepts — in the two shapes the frozen code matches: microVM's portable `*ExitStatusError` (`Signal != 0`) and podman's byte-identical `*exec.ExitError` (`ExitCode() == -1`). The error SHAPE is capability-gated (`portableKillError`) rather than forced to one type, because asserting `*ExitStatusError` on podman would require a forbidden production change to `PodmanCLI` and contradict record 590-591. This honors the frozen record.

### Verification

`gofmt -l` clean; `go build`/`go vet` clean untagged, under `-tags 'microvm unix'`, and under `-tags podman`; `go test -race -count=1 ./internal/runtime/...` ok; `golangci-lint run` 0 issues (untagged, and under each tag for the touched files). Live: `go test -tags podman -run TestContractSuite_Podman` PASS (all shared rows; microVM-gated rows correctly absent). Live on KVM: `go test -tags 'microvm unix' -run TestContractSuite_MicroVM` — all 15 rows PASS (the exec rows exercise the U2 guest PATH resolution and the whole suite the U4 socket-budget fix).

Ledger-impact: none

Spec-impact: none. Refs RIG-2493

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