Preserve overlapping input rows in locked append and consolidation

[justinjoy]

Summary

• Stage overlapping input rows before leased append COW or growth so the source remains readable through publication.
• Route incremental and k-way consolidation mutation through exact relation mutation leases and prepared radix sequences; remove the raw source-writer mutation bridges.
• Preserve outer-operation memory denial evidence across nested leased appends and document the added atomic operations.

Closes #2034.

Validation

• Full GCC POSIX ThreadSanitizer suite: 415 passed, 0 failed, 15 skipped (430 total).
• ASan with leak detection: relation_generations, radix_sort, and consolidate_kway_merge passed (3/3).
• Focused tests: relation_generations, relation_generations_oom, and consolidation coverage passed.
• Threading inventory: all 219 documented atomic sites match source; threading_doc and threading_doc_selftest pass.
• git diff --check passes.

Code size remains informational under the repository policy until a stable version is declared.