Fix flex-quant: use piscem map-sc subcommand (not map-scrna)

The released C++ piscem (bioconda) uses 'map-sc' as the single-cell
mapping subcommand. 'map-scrna' is only available in the Rust
piscem-rs development build. This caused flex-quant to fail with
"unrecognized subcommand 'map-scrna'" for users with the released
piscem.

Fixes #191.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rob-p

src/simpleaf_commands/flex_quant.rs

@@ -192,7 +192,7 @@ pub fn flex_map_and_quant(af_home: &Path, opts: FlexQuantOpts) -> anyhow::Result
     info!("Mapping reads with piscem...");
     let mut piscem_cmd = std::process::Command::new(&piscem_info.exe_path);
     piscem_cmd
-        .arg("map-scrna")
+        .arg("map-sc")
         .arg("-i").arg(&index_path)
         .arg("-g").arg(chem.geometry())
         .arg("-o").arg(&map_output)