Context

WT_DISAGG_SLOW_TRUNCATE_BUILD is a compile-time CMake flag that sets __wt_process.disagg_slow_truncate_2026 at startup, gating the old slow truncate code path. It was introduced as a safety valve while fast truncate was being stabilised on disaggregated storage.

Definition of Done

• Remove the #ifdef WT_DISAGG_SLOW_TRUNCATE_BUILD block in src/support/global.c and the disagg_slow_truncate_2026 field from src/include/connection.h
• In src/txn/txn_truncate.c, src/cursor/cur_layered.c, src/schema/schema_truncate.c, and src/include/txn_inline.h: delete the slow-path branches and the assertions that guard against the flag being set
• Remove the WT_DISAGG_SLOW_TRUNCATE_BUILD variable and its injection from test/evergreen.yml
• Confirm all fast-truncate tests pass without the flag