fix(arweave-scheduler): consume completed local prefills (marker-aware, local-only)

[xylophonez]

Summary

Adds an opt-in, marker-aware, local-only consumption path to the Arweave scheduler sync, so a node that has pre-populated a block range into its local store (via ~copycat@1.0 headers mode) indexes from that local data and never falls back to a remote fetch for a covered range.

Behaviour

• Opt-in. Gated by a scheduler-prefilled read policy; unset, behaviour is unchanged.
• Marker-gated. Block, transaction, and process-spawn header reads consult the local copycat block alias first. A miss after a committed per-block completion marker is a definite state ("intentionally not held"), not a trigger to reach remote — the remote fallback becomes unreachable for a marked range.
• Fail-closed. Missing or corrupt prefetched data fails closed rather than silently escaping to a gateway; a completion marker is only trusted because the copycat producer wrote it under fail-closed semantics.

Dependencies (for review)

• Base branch is candidate/fast-arweave-scheduler-1 — the dev_arweave_scheduler_sync device does not exist on edge yet, so this necessarily stacks on that candidate work.
• Runtime-depends on the copycat headers-mode + fail-closed completion change (fork PR for dev_copycat_arweave), which is what makes a completion marker a trustworthy guarantee. This scheduler change is only safe on top of that.

Scope

Single file: src/preloaded/process/dev_arweave_scheduler_sync.erl. No core/store/HTTP changes.

Motivation

Enables a resource-constrained node (e.g. a phone) to bootstrap its scheduler from a locally-verified, native-peer-fed block cache with a hard no-remote-escape guarantee, instead of racing public gateways per transaction.