policy: resolve every knob consumer through the shared override chain

Review fixes (ds-review-bot warnings on #623):

- One chain, every consumer: the override resolution (own post-seed
  switches ?? header baseline, closed-vocabulary validated) moves into pure
  exports (sandboxOverrideOf / approvalOverrideOf); the services delegate,
  and the permission presets consume them — current(session) and set()
  now see inherited baselines, so a child inheriting danger-full-access
  gets REAL knob switches when workspace-write is selected instead of a
  silent no-op, and a seed-carried preset selection is subsumed by the
  baseline. current(events) becomes current(session) (pre-release; the
  only callers were tests).
- Unconditional durable validation: a malformed header baseline fails
  loud on every read, no longer shadowed by an own switch.
- The two policy peers are declared optional (peerDependenciesMeta), so a
  thin spawn/fork deployment without policy plugins can consume the
  driver; verify-runtime-closure honors the flag.

Red-first: inherited-preset derive/switch-away and seeded-selection tests
in the permission suite; malformed-baseline-with-own-switch tests in both
policy suites.

packages/ui/user-approval/tests/approval.spec.ts

@@ -636,4 +636,12 @@ describe('delegation inheritance (overrideOf over the header baseline)', () => {
 
     expect(() => ctx.approval.overrideOf(child)).toThrow(/approvalPolicy/)
   })
+
+  it('rejects a malformed baseline even when an own switch would win (validation is unconditional)', async () => {
+    const ctx = await mounted()
+    const child = inheritedSession('sess-appr-invalid-own', { approvalPolicy: 'always' })
+    setApprovalPolicy(child, 'never')
+
+    expect(() => ctx.approval.overrideOf(child)).toThrow(/approvalPolicy/)
+  })
 })