Function fromModule

Module-as-Source: treat a module selector as an ExternalStore source (IR-recognizable dependency).

Notes:

• moduleId must be resolvable; passing a read-only ModuleHandle is forbidden (fail-fast).
• selectorId must be stable: ReadQuery.compile(selector) must NOT fall back to fallbackReason="unstableSelectorId" (fail-fast).
• Value semantics: the runtime does not clone; writeback stores the selector return value as-is (keep selectors small and stable).