refactor: optimize MPHF construction and update legacy guidelines

Replaces parallel random-access unitig iteration with a sequential mmap-based iterator for MPHF construction, eliminating the build-time `.idx` dependency by deferring index generation until after persistence. Updates `CLAUDE.md` to treat existing code as a hypothesis, mandating proactive removal of obsolete legacy constructs rather than preserving them out of inertia.

src/obilayeredmap/src/mphf_layer.rs

@@ -218,11 +218,15 @@ impl MphfLayer {
 
         match evidence_kind {
             // ── Exact path ────────────────────────────────────────────────────
+            // .idx is built LAST, once evidence.bin is written, so it is never
+            // present during construction — only at query time.
             EvidenceKind::Exact => {
-                build_unitig_idx(&unitig_path, block_bits)?;
-
-                let unitigs = UnitigFileReader::open(&unitig_path)?;
-                let n = unitigs.n_kmers();
+                let n = UnitigFileReader::open_sequential(&unitig_path)?.n_kmers();
+                let keys = CanonicalKmerIter::new(&unitig_path)
+                    .map_err(|e| match e {
+                        obiskio::SKError::Io(io) => OLMError::Io(io),
+                        e => OLMError::InvalidLayer(e.to_string()),
+                    })?;
 
                 if n == 0 {
                     fs::File::create(dir.join(EVIDENCE_FILE))?;
@@ -232,15 +236,13 @@ impl MphfLayer {
                     mphf.store(&dir.join(MPHF_FILE))
                         .map_err(|e| OLMError::InvalidLayer(e.to_string()))?;
                     LayerMeta::exact().save(dir)?;
+                    build_unitig_idx(&unitig_path, block_bits)?;
                     return Ok(0);
                 }
 
-                // Pass 1 — parallel MPHF via random access (.idx required)
-                let keys = (0..unitigs.len())
-                    .into_par_iter()
-                    .flat_map_iter(|ci| unitigs.unitig(ci).into_canonical_kmers().map(|km| km.raw()));
+                // Pass 1 — MPHF construction via clonable mmap iterator
                 let mphf: Mphf =
-                    Mphf::new_from_par_iter(n, keys, PtrHashParams::<CubicEps>::default());
+                    Mphf::new_from_par_iter(n, keys.map(|k| k.raw()).par_bridge(), PtrHashParams::<CubicEps>::default());
                 mphf.store(&dir.join(MPHF_FILE))
                     .map_err(|e| OLMError::InvalidLayer(e.to_string()))?;
 
@@ -266,6 +268,8 @@ impl MphfLayer {
 
                 ev.write(&dir.join(EVIDENCE_FILE))?;
                 LayerMeta::exact().save(dir)?;
+                // .idx built last: strictly for query-time kmer verification
+                build_unitig_idx(&unitig_path, block_bits)?;
                 Ok(n)
             }