feat: add evidence metadata and configurable k-mer parameters

Introduces `EvidenceKind` and `LayerMeta` structs to manage per-layer evidence configuration and false-positive parameters. Adds JSON serialization for layer metadata persistence and updates `build_approx_evidence` to accept a `z` parameter for consecutive k-mer thresholds. Exposes these types publicly and documents a future `aggregate` command for merging index matrix columns.

src/obikindex/src/meta.rs

@@ -3,6 +3,7 @@ use std::fs;
 use std::io;
 use std::path::Path;
 
+use obilayeredmap::EvidenceKind;
 use serde::{Deserialize, Serialize};
 
 pub const META_FILENAME: &str = "index.meta";
@@ -28,6 +29,8 @@ pub struct IndexConfig {
     pub minimizer_size: usize,
     pub n_bits: usize,
     pub with_counts: bool,
+    #[serde(default)]
+    pub evidence: EvidenceKind,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]