POC series T0a → T0b → T0c (~10 days total, ~2 weeks). This ticket is T0b (~4 days).

Changes:

• Rough _bulk_reconcile_suspend() / _bulk_reconcile_resume() — preserve key fields across flush cycles (final field list determined by T2)
• Rough __bulk_find_rightmost_internal() — O(log N) walk from root
• Rough __bulk_attach_rightmost_leaf() — append child, publish new WT_PAGE_INDEX

Goal: Both hard components work well enough to wire together in T0c. Production quality not required — proof of design feasibility is the goal.

Depends on: WT-17209 (T0a)
Next: T0c — rough orchestrator + smoke test