Backends and execution — the domain

Who runs the prompt, with which tools, and what the node keeps of its native harness. Leaves, one line each:

Leaf	Read it when
backend-selection.md	Wrong backend or wiring a new one.
Capability registry	Backend facts, unchanged seed, derived enumerations and mutation witnesses.
gateway.md	Serving openai_compatible/… from the deployment env, its catalog, its refusals, where provider/model config lives.
fallbacks-and-parity.md	fallbacks:, C173/C176, a "dumber" node, a degrading forfait.
facade-substitution.md	GLM/Kimi served under a claude/gpt label, or a base URL the substitution verdict ignores.
metering.md	Trusted delegate counters, reserved output metadata, interaction spend.
plugins-compression.md	Output compression (rtk), secret substitution, or a plugin changing what a node sees.
sandbox.md	SANDBOX_* and ATTACHMENT* failures, host binds dropped or carried, changing what a node may execute.
prompt-engineering-dials.md	Authoring cursor: / supervisor: / ultracode.