refactor: implement RoutableSuperKmer and update k-mer indexing pipeline

Replace raw SuperkMer routing with a new RoutableSuperKimer type that embeds canonical sequences and precomputed minimizers, enabling direct partition routing via hash. Update the build pipeline to yield RoutableSuperKmers throughout (builder, scatterer), refactor FASTA/unitig export commands to use the new type and compressed outputs (.fasta.gz, .unitigs.fasta.zst), revise SuperKmer header to store n_kmers instead of seql (avoiding 256-byte wrap), and update documentation to reflect minimizer-based theory, two evidence-encoding strategies for unitig-MPHF indexing (global offset vs. ID+rank), and the new obipipeline library architecture with parallel workers, biased scheduling, and error handling.

mkdocs.yml

@@ -16,7 +16,11 @@ markdown_extensions:
   - admonition
   - footnotes
   - tables
-  - pymdownx.superfences
+  - pymdownx.superfences:
+      custom_fences:
+        - name: mermaid
+          class: mermaid
+          format: !!python/name:pymdownx.superfences.fence_code_format
   - pymdownx.arithmatex:
       generic: true
 
@@ -39,6 +43,7 @@ nav:
       - obipipeline library: implementation/obipipeline.md
       - On-disk storage: implementation/storage.md
       - MPHF selection: implementation/mphf.md
+      - Unitig evidence encoding: implementation/unitig_evidence.md
   - Architecture:
       - Sequences: architecture/sequences/invariant.md