* test: run each heavy unit test file in its own process
`test:unit` and `test:unit:heavy` ran every file in one `bun test`
process. A heavy fixture file alone takes 2-6 GB, and what one file
keeps stays alive for the rest of the run. Quick files still share one
process; heavy files now run one per process, so the peak is the
heaviest single file. Coverage runs keep one process, since coverage is
collected per process.
stale-overrides parses material3.fig and joins the heavy list, and a
guard fails when a quick test reads a large fixture corpus.
* test(core): keep one material3 graph alive at a time in stale-overrides
The edited-document test held the exported reader graph, the reopened
graph, and the parsed node changes at once, and the first test's graph
was still alive when the second started. Each step now runs in its own
function and returns only what the next check needs, and the file
collects garbage between steps and tests. The checks are unchanged;
peak memory drops from 5.6 GB to 3.6 GB.
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