refactor: add rolling buffer methods and document label constraints

Added `is_empty()`, `clear()`, and `iter()` methods to the rolling statistics buffer to enable standard traversal and state reset operations. Documented genome label constraints, specifying forbidden characters, empty label rejection, space quoting requirements, and auto-derived label bypass rules. Additionally, updated doc comments and added `#[allow(dead_code)]` attributes for `kmer_offset` and `n_kmers` fields to suppress compiler warnings while reserving them for future `--detail` coverage vector logic.

docmd/index.md

@@ -17,7 +17,7 @@
 | `unitig`    | Dump unitigs from a built index to stdout (debug) |
 | `estimate`  | Estimate approximate-index parameters (z, evidence bits, FP rates) before indexing |
 | `reindex`   | Convert an index's evidence in-place: exact ↔ approx |
-| `utils`     | Miscellaneous index utilities: `--new-label NEW=OLD` renames a genome label in-place |
+| `utils`     | Miscellaneous index utilities: `--new-label NEW=OLD` renames a genome label in-place (NEW gets OLD's identity) |
 
 ## Constraints
 
@@ -27,6 +27,20 @@
 - Canonical form: `min(kmer, revcomp(kmer))` reduces strand-symmetric space by half
 - Input formats: FASTA, FASTQ, gzip, streaming stdin; `index` reads from stdin automatically when no input files are provided (`-` can also be passed explicitly among other paths)
 
+## Genome label constraints
+
+Genome labels are arbitrary Unicode strings with the following restrictions:
+
+| Character | Forbidden | Reason |
+|-----------|-----------|--------|
+| `/` | yes | filesystem path separator |
+| `=` | yes | `--new-label` parser separator |
+| `\0` | yes | null byte |
+| `\n` `\r` `\t` | yes | break CSV output |
+| spaces | **allowed** | use shell quoting: `--new-label 'new label=old label'` |
+
+Empty labels are also rejected. Labels derived automatically from the index directory name (when `--label` is omitted) are not validated since they come from the filesystem and are already safe.
+
 ## Priority operations
 
 - Kmer counting (frequencies)