feat: add utils subcommand for renaming genome labels

Introduces a `utils` CLI subcommand to enable in-place genome label renaming without full reindexing. Adds strict label validation to reject empty strings, filesystem separators, and control characters, ensuring safe CSV serialization. Updates index metadata, renames corresponding spectrum JSON files, and registers the command in the main dispatch logic. CLI reference documentation is also updated.

docmd/index.md

@@ -17,6 +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 |
 
 ## Constraints
 
@@ -24,7 +25,7 @@
 - Maximum efficiency in computation, memory, and disk usage
 - k odd, k ∈ [11, 31], fixed at runtime; kmer fits in a u64 (2 bits/base)
 - Canonical form: `min(kmer, revcomp(kmer))` reduces strand-symmetric space by half
-- Input formats: FASTA, FASTQ, gzip, streaming stdin
+- 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)
 
 ## Priority operations