feat(sandbox): the confinement seam and the per-platform native runner chains

ctx.sandbox (dsh-sandbox): confine(argv, policy) returns the argv to spawn
instead — wrapped so the process and its children run confined — plus the
enforcement completeness and the backend denial/runner-failure dialects;
no usable backend throws the fail-closed SANDBOX_UNAVAILABLE. Policy rides
per call. dsh-sandbox-local selects by platform and caches the verdict:
multi-candidate chains probe FUNCTIONALLY in preference order (Linux:
bwrap → the registry-installed node-addon-landlock-run launcher), a sole
candidate is selected unprobed (darwin: sandbox-exec/Seatbelt) and fails
closed at execution via runnerFailureSignatures; win32 is a reserved empty
chain. Profile parity is honest per backend (documented temp-area and ABI
differences; enforcement full|partial is a structured result fact).

CI: the sandbox-e2e matrix proves real-kernel confinement per rung (bwrap,
Landlock per architecture through the registry-installed launcher,
Seatbelt), failing on a silent all-skip; the packed-install rehearsal
installs the launcher family from the registry and asserts the binary
executable apart from kernel enforcement.

tsdown.config.ts

@@ -12,7 +12,8 @@ import { defineConfig } from 'tsdown'
 export default defineConfig({
   // Explicit globs: `workspace: true` would also discover examples (any
   // package.json), but only vendor and the packages hierarchy are pnpm
-  // workspaces.
+  // workspaces. The Landlock launcher platform packages ship a prebuilt
+  // native binary and no JavaScript — nothing to bundle.
   workspace: ['vendor/*', 'packages/*/*'],
   entry: ['lib/types/index.js'],
   outDir: 'lib',