MEM-018: graf walkers carve BASS per step without rewind (graf/BLAME.c, DIFFREF.c, BRAM.c)

Now
DONE

Three graf walkers carve ABC_BASS scratch inside per-step/recursive helpers that are invoked by plain call (no call()/try() and no mark/rewind), so the scratch accumulates across the walk instead of dying per iteration; on large inputs the 1GB arena is exhausted, after which a_carve returns NOROOM, the work silently truncates (some via a (void) cast), and a misleading "auto-rewound at the caller's boundary" comment masks it. The goal is to rewind per iteration. (REBASE's variant is MEM-008.)

Issues

Per-iteration carves outlive the iteration.

Blockers

None. Graceful NOROOM (no OOB), but silent truncation + arena exhaustion.

Planned

Rewind around each iteration/descent; fix the comments.