Part of WT-18574, on the block-cache-depr feature branch. Test-only: src/ is untouched. The only legacy consumer in test/suite is test_layered_config02.py (long delta chains), currently skipped in early_setup() under FIXME-WT-15663. The skip, the legacy config and the block_cache_blocks_removed assertion must all go before S2 closes WT-15663 and deletes the counter.

Work

• Remove the early_setup() skip, the block_cache=(enabled=true,...) config and the block_cache_blocks_removed assertion from test_layered_config02.py; keep the long-delta-chain coverage on the default configuration, or delete the file with rationale if nothing separable remains.
• Keep victim-cache coverage: test_layered_victim_cache01/02.py, test/catch2/misc_tests/test_evict_disagg_cache_eligible.cpp, test/catch2/block/util_block.cpp.
• Grep test/ for stat-enumeration/count/ordering tests and check them against the 22 legacy counters (deleted in S1/S2 with their last updater). The seven victim-cache block_cache_* counters stay.

Acceptance

• No test enables WT_BLKCACHE or asserts a deleted legacy statistic; the FIXME-WT-15663 skip is gone.
• Shared block I/O and victim-cache coverage remain; no deprecation-behavior assertions yet (those land with S1).
• No src/ changes.

Dependencies

Parallel with T1. Hard: merge before S2 — FIXMEs must reference open tickets, and the counter is deleted there. Before S1 to keep the phase sequence simple.