docs: expand kmer indexing, filtering, and merging documentation

Expands MkDocs navigation and documentation for evidence elimination, the merge command, and kmer filtering. Refactors kmer representation to a generic `KmerOf<L>` type with a bitwise reverse complement algorithm. Unifies MPHF construction, introduces approximate fingerprint-based indexing, and updates the pipeline, chunkreader, and storage layouts. Adds code coverage reports and clarifies architectural invariants for improved maintainability.

doc/kmers/index.html

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+      <a href="../implementation/evidence_elimination/" class="md-nav__link">
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+    Evidence elimination (discussion)
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+      <a href="../implementation/merge/" class="md-nav__link">
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+    Merge command
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+      <a href="../implementation/rebuild_filter/" class="md-nav__link">
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+    Kmer filtering (rebuild/dump/unitig)
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@@ -1038,11 +1122,12 @@
 <h2 id="kmers">Kmers</h2>
 <p>A <strong>kmer</strong> is a DNA subsequence of fixed length k. Two constraints govern the choice of k:</p>
 <ul>
-<li><strong>k ∈ [11, 31]</strong>: the range ensures the kmer is long enough to be specific and short enough to fit in a single machine word.</li>
+<li><strong>k ∈ [11, 31]</strong>: the range ensures the kmer is long enough to be specific and short enough to fit in a single machine word (u64 at 2 bits/base requires k ≤ 32; k &lt; 11 yields insufficient specificity).</li>
 <li><strong>k is odd</strong>: an odd-length sequence cannot equal its own reverse complement (no palindromes). This guarantees that the canonical form <code>min(kmer, revcomp(kmer))</code> is always strictly defined — the two orientations are always distinct — which is required for strand-independent counting.</li>
 </ul>
+<p>Both constraints are <strong>enforced at CLI entry</strong> by <code>CommonArgs::validate()</code> in <code>superkmer</code> and <code>index</code>. Passing an invalid k exits immediately with an error message.</p>
 <h2 id="super-kmers">Super-kmers</h2>
-<p>A <strong>super-kmer</strong> is a maximal run of consecutive kmers from a DNA read, each overlapping the next by k−1 nucleotides. Each kmer of the run carries the same <strong>canonical minimizer</strong>. The <strong>canonical minimizer</strong> of a kmer is the smallest value of <code>min(m-mer, revcomp(m-mer))</code> over all m-mers within the kmer (m &lt; k, m odd), with the constraint that <strong>non-degenerate m-mers are always preferred</strong> over degenerate ones. A degenerate m-mer is one composed of a single repeated nucleotide (all-A, all-C, all-G, or all-T); such m-mers are selected only if no non-degenerate candidate exists in the window.</p>
+<p>A <strong>super-kmer</strong> is a maximal run of consecutive kmers from a DNA read, each overlapping the next by k−1 nucleotides, sharing the same <strong>canonical minimizer</strong>. The <strong>canonical minimizer</strong> of a kmer is the m-mer (m &lt; k) whose canonical hash <code>hash_kmer(min(m-mer, revcomp(m-mer)))</code> is smallest over all m-mers in the kmer window. The hash function is a <code>mix64</code>-based bijection; selection is purely hash-ordered with no degeneracy filter. A super-kmer is capped at 256 nucleotides; a longer run is split at that boundary.</p>
 <h3 id="canonical-super-kmers">Canonical super-kmers</h3>
 <p>A <strong>canonical super-kmer</strong> is the lexicographic minimum of a super-kmer and its reverse complement:</p>
 <div class="highlight"><pre><span></span><code>canonical(super-kmer) = min(super-kmer, revcomp(super-kmer))