Handoff sidecar reader: canon says a non-integral pid is refused, the code admits it #353

Canon and code disagree on how the persistent-terminal handoff sidecar reader treats a pid that is not an integer.

• Canon: docs/architecture/client-runtime.md (Background service, "Stricter readers than upstream's JSON.parse") says "The sidecar reader refuses a pid that is not integral. A refused document reads as absent."
• Code: src/OpenCode.Sdk/Hosting/BackgroundService/Internal/Handoff/PtyHandoffSidecar.cs (around lines 149-160) admits such a document with SourcePid = null.
• Tests require the code's behaviour: tests/OpenCode.Sdk.Tests/Hosting/BackgroundService/Handoff/PtyHandoffSidecarTests.cs (around lines 59-85) expects admission, and ServicePtyHandoffTests.cs (around lines 246-254) expects adoption without a readable registration.

One of the two is wrong. Under the deviation protocol, this needs a decision on which behaviour is intended, checked against upstream's pinned packages/client/src/pty-handoff.ts, before either side changes.

Found by the Faz 4 RT-E1 implementation plan (2026-10-16), at origin/master eef754bd. RT-E1's U4-15 fix (whole-document reads) does not touch this behaviour.