feat: enforce runtime validation for kmer and minimizer parameters

Introduces `CommonArgs::validate()` to enforce strict constraints on `--kmer-size` (odd, 11–31), `--minimizer-size` (odd, 3–k−1), and `z` (strictly less than k). This validation is applied at the entry point of the `superkmer` and `index` commands to prevent invalid configurations, avoid palindromes, prevent u64 overflow, and ensure positive effective indexing sizes. Documentation is updated to reflect these runtime checks and immediate termination on invalid input.

docmd/kmers.md

@@ -4,9 +4,11 @@
 
 A **kmer** is a DNA subsequence of fixed length k. Two constraints govern the choice of k:
 
-- **k ∈ [11, 31]**: the range ensures the kmer is long enough to be specific and short enough to fit in a single machine word.
+- **k ∈ [11, 31]**: 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 < 11 yields insufficient specificity).
 - **k is odd**: an odd-length sequence cannot equal its own reverse complement (no palindromes). This guarantees that the canonical form `min(kmer, revcomp(kmer))` is always strictly defined — the two orientations are always distinct — which is required for strand-independent counting.
 
+Both constraints are **enforced at CLI entry** by `CommonArgs::validate()` in `superkmer` and `index`. Passing an invalid k exits immediately with an error message.
+
 ## Super-kmers
 
 A **super-kmer** is a maximal run of consecutive kmers from a DNA read, each overlapping the next by k−1 nucleotides, sharing the same **canonical minimizer**. The **canonical minimizer** of a kmer is the m-mer (m < k) whose canonical hash `hash_kmer(min(m-mer, revcomp(m-mer)))` is smallest over all m-mers in the kmer window. The hash function is a `mix64`-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.