Enable streaming writes automatically

Enable streaming writes when a stencil writes an element w/o a preceding read.

The issue names no files, tests, or entry points. Start by locating the stencil write-analysis and streaming-write decision logic, then trace the case where an element is written without a preceding read. Done means streaming writes are enabled automatically for that case, with coverage in the relevant tests if such tests exist.

Written by the indexing model from the issue text.