Arm B serves pre-baked structure too

Eli: "Put all of the features in the B arm. that's what I asked for."

Fair. I built the sidecars and then left them unconsumed, which made arm B
the whole bet minus its newest piece.

The library handlers now resolve structure through `documentStructure` /
`documentGraph`, which read a pre-baked sidecar in the address arm and compile
synchronously in legacy. A miss falls through to a real compile, so a document
nobody baked behaves exactly as before.

The blocker I cited for not doing this was that `bakedSkeleton` is async where
`compileAgreementSkeleton` is sync. It was not a real blocker: every one of
these handlers is already inside an async function that awaits
`extractLocalDocument`. Awaiting costs nothing there.

Coding-shape Read/Edit/Grep keep the synchronous path deliberately -- that is a
separate surface with its own A/B, and changing it here would confound two
experiments.

103 tests green; both arms typecheck.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H9ToHYJVDxfeJcJwdzrP2H