refactor: migrate pipeline to NucPage-based stream processing

Replace the existing chunk and Rope-based processing pipeline with a fixed-size NucPage architecture. Introduce a new nucstream module featuring buffer-pooled, in-place parsing that auto-detects and decompresses FASTA/FASTQ/GenBank inputs into normalized ACGT streams with k-mer overlap preservation. Update obikmer scatter and superkmer stages to consume NucPage iterators and cursor-based navigation, eliminating std::io::Read dependencies and optimizing memory management. Add a configurable max_open_files CLI argument and update implementation documentation to reflect the new record vs. stream reading paths.

docmd/implementation/pipeline.md

@@ -19,7 +19,11 @@ The histogram gives:
 
 ## Phase 1 — Scatter
 
-Single streaming pass over raw input files (FASTA/FASTQ, gzip). FASTQ quality scores are ignored. For each read:
+Single streaming pass over raw input files (FASTA/FASTQ, gzip). FASTQ quality scores are ignored.
+
+Input files are read via `open_nuc_stream`, which opens and decompresses the file, auto-detects the format (FASTA / FASTQ / GenBank), and yields a sequence of `NucPage` buffers. Each `NucPage` is a flat 64 KB buffer of normalised bytes (`ACGT` + `\x00` separators), carrying a k−1 byte overlap from the preceding page so that no k-mer is lost at page boundaries. Per-record identity (sequence id, raw bytes) is not preserved; this is intentional — the scatter phase only needs normalised bases to produce superkmers.
+
+For each read fragment within a page:
 
 1. **Ambiguous base filter**: cut at any non-ACGT base; discard fragments shorter than k.
 2. **Entropy filter**: scan each fragment with a sliding window of size k. When the kmer $K_i = S[i \mathinner{..} i+k-1]$ ended by nucleotide $S[j]$ (with $j = i+k-1$) has entropy below threshold $\theta$, emit the current segment and start a new one (see algorithm below). $K_i$ belongs to neither segment, and no valid kmer is lost.