Makes codegraph_node a drop-in faster Read for indexed source files (file-read mode: <n>\t<line> like Read, offset/limit, + blast-radius header; symbolsOnly for the map). Fixes the old file-view dropping imports/line-numbers. #383/#527 preserved. Validated by A/B: explore/node already return source + line numbers, so Read=0 when used. Includes the A/B eval harness scripts. Full suite green (1270).
