[Design] Row-level concurrency for OSS Delta — superseded by umbrella #3 + per-case issues #1

Superseded. With the umbrella (#3) as the top-level tracker and each case carrying its own design in
its own issue, a separate full-design issue is no longer needed:

• [RLC] Case 1 — conflict-time data skipping (disjoint data ranges) #4 — Case 1: reader-side conflict-time data skipping
• [RLC] Case 2 — same-file deletion-vector merge (concurrent DML ⟂ DML) #5 — Case 2: same-file deletion-vector merge (DML ⟂ DML)
• [RLC] Case 3 — exclude relocation (dataChange=false) files from the append-conflict check #6 — Case 3: OPTIMIZE vs non-blind append (dataChange=false)
• [RLC] Case 4 — OPTIMIZE ⟂ DML reconciliation (compaction offset remap) #7 — Case 4: OPTIMIZE vs DML reconciliation (compaction offset remap; reclustering via row tracking)

See #3 for the overview and organizing principle, and the per-case issues above for design detail.