feat(copycat): headers-only indexing mode with fail-closed completion

[xylophonez]

Summary

Adds a headers-only indexing mode to the ~copycat@1.0 Arweave engine (dev_copycat_arweave), plus fail-closed completion semantics for it.

Headers mode replicates a block range using one block2 layout request per block and fetches individual transaction headers only for the zero-data transactions that require them, instead of pulling every transaction. For low-fanout consumers — resource-constrained or edge nodes backfilling a large historical range from the node's native Arweave peer pool — this collapses the request count dramatically (a typical tip block resolves in ~2 requests instead of one-per-transaction) while writing the same canonical header/index entries as the existing path.

Behaviour

• Opt-in. Existing copycat modes are unchanged; headers mode is selected explicitly. Default behaviour is identical to before.
• Fail-closed. A malformed or mismatched block2 layout, an out-of-order/incorrect block id, or a header that cannot be retrieved is treated as a hard failure. The engine never returns a success status for a partially-indexed range and never writes a per-block completion marker after a skipped header. A completion marker therefore means "every required header for this block is committed locally," so downstream consumers can trust it as a real guarantee rather than a best-effort hint.
• No silent remote fallback. Header/layout retrieval stays on the node's configured native routes; a missing local entry after a committed marker is a definite state ("data-bearing, intentionally not held"), not a trigger to reach upstream.

Scope

Single file: src/preloaded/query/dev_copycat_arweave.erl. No changes to core, stores, HTTP, or other devices.

Testing

Covered by the copycat package suite, including an HTTP-level fail-closed test that drives a valid block, a malformed block2, and an invalid backing store through the real packaged HTTP path and asserts a non-2xx result with no completion marker written while still only touching the native block/block2 routes.