docs(adr): ADR-0076 rev.4 — relocate protocol to @objectstack/metadata-protocol (it's metadata-domain, mis-located in objectql)

Corrects the centerpiece. protocol.ts implements ObjectStackProtocol (contract
in spec), manages sys_metadata/locks/commits/ownership — metadata-domain work
that only uses the engine as storage via a 5-method SysMetadataEngine interface.

Measured: protocol.ts co-changes with engine.ts only 6% of the time vs 43% with
metadata-core/metadata/spec; move blast radius = 2 source files (plugin.ts,
index.ts). So the right "now" move is relocating protocol into a new
@objectstack/metadata-protocol package — not the rev.3 ./core subpath.

objectql becomes a lean data engine by construction (268KB leaves the package);
gateway depends on it with no protocol. Boundary ratchet keeps it pure.
Engine repo-split stays a separate, trigger-gated (88% cross-package) future phase.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: os-zhuang