Use lab SDSL v3 while preserving existing index formats
Use the lab SDSL v3 fork while preserving existing MetaGraph index formats and mmap loading. Replace the compiled-library setup with header-only includes and adapt the changed constructors and footprint accounting.
The fork also normalizes unused interleaved-vector bits before serialization, caches diagnostic type names to avoid repeated demangling during BRWT construction, and reduces per-vector object overhead. On the tested x86-64 build, int_vector is 48 bytes versus 40 in v2; sd_vector is 680 versus 600; rrr_vector<63> is 200 versus 168. Existing serialized layouts are preserved; dependents must be rebuilt.
Validation: 5,760 fixture reads and 80 real-index query runs pass across v2/v3 files and ordinary/mmap readers. Local BRWT construction medians recovered from 2.16 to 1.69 seconds for 10,000 reads and from 5.19 to 4.73 seconds for E. coli; these are small, warm-cache workloads, not production-scale benchmarks. Full CI will rerun for the updated fork.
Review/merge order: #648 → #649 → #646 → #644. #647 is already merged.