fix: validate the snapshot that installs, not the object that was shown

Three trust boundaries checked a caller's object and then read it again to
use it. Between those two reads an accessor or a Proxy can answer
differently, so the value that passed validation and the value that was
installed were not the same value.

A credential owner's answer was read field by field outside the auth
boundary: a throwing `kind` getter escaped into the transport catch and an
auth outage reached operators as `NETWORK_FAILURE`. Contract composition
validated a contribution and then copied it, so a policy that answered
10,000 to the ceiling check and 999,999 to the copy installed the second
value. The cursor runtime validated its profile once and re-read it on
every page, so raising `maxPages` after construction widened a cap that
had already been checked.

`src/contracts/exact-snapshot.ts` is the one descriptor-based decoder they
now share: every property is read exactly once, an accessor, a symbol, an
inherited or non-enumerable field and a throwing trap all resolve to a
typed failure, and validation runs on the owned copy.

Separately, the `responseBody: NONE` probe awaited a bare `read()`. The
deadline produced a bounded public result while the raw reader kept its
lease, so the body stayed locked and the outer compensator could not
cancel it. The probe now takes the operation lifetime and owns the cancel
and the lock release itself.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/integration/http-execution-contract.test.ts

@@ -71,7 +71,6 @@ describe("HTTP operation execution contract", () => {
     const attachCredentials = vi.fn(() => ({
       kind: "READY" as const,
       headers: {},
-      credentials: "omit" as const,
     }));
     const observedKeys: Array<string | null> = [];
     const fetcher = vi.fn(