🔧 Replace degenerate minimizer logic with hash-based random ordering

- Add `hash` field to MmerItem for stable, randomized minimizer ordering
- Introduce hash_mMER() using mix64 with XOR seed to avoid fixed points (e.g., poly-A/T)
- Remove is_degenerate() and minimizer_worse(), simplifying comparison to hash-only
- Update push logic: compare hashes instead of canonical values with degeneracy checks

docmd/implementation/superkmer.md

@@ -82,6 +82,50 @@ bits[len - shift..].fill(false)
         return seq                                            -- palindrome: either orientation valid
     ```
 
+## Minimizer sliding window
+
+Super-kmers are built by `SuperKmerIter` (crate `obiskbuilder`), which maintains the current minimizer with a **monotonic deque** over a sliding window of W = k − m + 1 m-mer positions.
+
+Each deque entry stores:
+
+| Field      | Type  | Purpose                                      |
+|------------|-------|----------------------------------------------|
+| `position` | usize | 0-based start of this m-mer in the segment   |
+| `canonical`| u64   | right-aligned canonical m-mer value (lex-min of fwd and rc); used as partition key |
+| `hash`     | u64   | $H(\text{canonical})$ — ordering key for random minimizer selection |
+
+The hash $H$ is the seeded splitmix64 finalizer (see [Minimizer selection](../theory/minimizer.md)):
+
+```rust
+fn hash_mmer(canonical: u64) -> u64 {
+    let x = canonical ^ 0x9e3779b97f4a7c15;   // seed: eliminates fixed point at 0
+    let x = x ^ (x >> 30);
+    let x = x.wrapping_mul(0xbf58476d1ce4e5b9);
+    let x = x ^ (x >> 27);
+    let x = x.wrapping_mul(0x94d049bb133111eb);
+    x ^ (x >> 31)
+}
+```
+
+On each new nucleotide, once the window is full, the deque is updated:
+
+!!! abstract "Algorithm — minimizer deque update"
+    ```text
+    procedure UpdateMinimizer(deque, position, canonical, hash, k, received):
+        -- pop dominated entries from the back
+        while deque.back.hash ≥ hash:
+            deque.pop_back()
+        deque.push_back({position, canonical, hash})
+
+        -- evict expired entries from the front
+        while deque.front.position + k < received:
+            deque.pop_front()
+    ```
+
+The front of the deque is always the current minimizer. Because the deque is maintained in strictly increasing hash order, each entry is popped at most once — O(1) amortized per nucleotide.
+
+A super-kmer boundary is emitted when the minimizer changes: `deque.front.hash ≠ prev_hash`. The `canonical` field of the front entry is **not** used for boundary detection — that uses the hash alone. The canonical value is stored so that the partition key $H(\text{canonical})$ can be recomputed independently at routing time from the stored `minimizer_pos`, without inheriting the minimum-order-statistic bias (see [Minimizer selection — partition key independence](../theory/minimizer.md#partition-key-independence)).
+
 ## Kmer extraction
 
 A k-mer is extracted from a super-kmer with `SuperKmer::kmer(i, k)`, which returns a `Kmer` — a left-aligned `u64` newtype (see [Kmer implementation](kmer.md)):