test_layered_fast_truncate18 is the only conflict test we have, and it is scoped to truncate against truncate. The stress test deliberately steers around conflicts entirely: its round schedule filters out any insert, update, or remove whose key falls inside an in-flight truncate range.

Add coverage for:

• a write landing inside a pending truncate range, and a truncate covering a pending write, in both orders
• readers positioned over a pending truncate range
• rollback of a truncate that overlaps concurrent writes

Then remove the conflict avoidance filter from the stress test so it exercises the real thing.

Depends on WT-17637.