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

Scope maps to "First implementation batch: minimal code cut" in implementation_details.md.

Changes:

• src/include/cache.h: add WT_SYNC_BULK_FLUSH to WT_CACHE_OP
• src/evict/evict.h: add WT_EVICT_CALL_BULK_RIGHTMOST_PATH
• src/reconcile/reconcile.h: add WT_REC_BULK_FLUSH
• src/reconcile/reconcile_private.h: add on_rightmost_internal_path field to WTI_RECONCILE
• src/evict/evict_file.c: walk helper + on_rightmost_internal_path computation
• src/evict/evict_page.c: translate evict-call context → reconcile context
• src/reconcile/rec_write.c: retain multi->disk_image only for last block of rightmost internal page

Goal: Phase 2 rightmost-path identification and image retention works. No resume logic, no tree structure changes. Reviewable independently.

Depends on: none
Next: T0b — rough reconcile suspend/resume + rightmost path rebuild