POC series T0a → T0b → T0c (~10 days total, ~2 weeks). This ticket is T0c (~3 days).

Changes:

• Rough __wt_bulk_flush() wiring all three phases from T0b
• __wt_evict_file(): add WT_SYNC_BULK_FLUSH case (rough)
• _curbulk_insert_row(): call _wt_bulk_flush() when threshold fires

Goal: Smoke test passes — single flush on a row-store table, final tree is logically correct (key/value contents and scan order match the no-flush mode).

Depends on: WT-17210 (T0b)
Next: T1 — configuration params, statistics, enum/flag production hardening