feat: centralize index configuration and add hybrid mode

Centralizes index configuration by storing a single `IndexMode` (`Exact`, `Approx`, or `Hybrid`) in `PartitionMeta`, eliminating per-layer metadata files. Introduces a `Hybrid` evidence mode and an `--approx` CLI flag to toggle between exact and probabilistic indexing. Refactors the build and query pipelines to dynamically dispatch based on the configured mode, deferring `.idx` generation to Pass 2 and only requiring it for Exact/Hybrid modes. Updates layer opening to load appropriate data structures, enforces strict parameter validation during merges, and clarifies performance trade-offs in documentation.

src/obikindex/src/merge.rs

@@ -9,7 +9,7 @@ use obisys::{Reporter, Stage};
 use rayon::prelude::*;
 use tracing::info;
 
-use obilayeredmap::EvidenceKind;
+use obilayeredmap::IndexMode;
 
 use crate::error::{OKIError, OKIResult};
 use crate::index::KmerIndex;
@@ -271,14 +271,16 @@ fn partition_bar(n: u64) -> ProgressBar {
 /// - all `Exact`                        → OK, returns `Exact`
 /// - all `Approx { b, z }` same params  → OK, returns `Approx { b, z }`
 /// - mixed exact/approx or different approx params → `IncompatibleEvidence`
-fn validate_evidence_compat(sources: &[&KmerIndex]) -> OKIResult<EvidenceKind> {
+fn validate_evidence_compat(sources: &[&KmerIndex]) -> OKIResult<IndexMode> {
     let ref_ev = &sources[0].meta.config.evidence;
     for src in &sources[1..] {
         let ev = &src.meta.config.evidence;
         let compat = match (ref_ev, ev) {
-            (EvidenceKind::Exact, EvidenceKind::Exact) => true,
-            (EvidenceKind::Approx { b: b1, z: z1 },
-             EvidenceKind::Approx { b: b2, z: z2 }) => b1 == b2 && z1 == z2,
+            (IndexMode::Exact, IndexMode::Exact) => true,
+            (IndexMode::Approx { b: b1, z: z1 },
+             IndexMode::Approx { b: b2, z: z2 }) => b1 == b2 && z1 == z2,
+            (IndexMode::Hybrid { b: b1, z: z1 },
+             IndexMode::Hybrid { b: b2, z: z2 }) => b1 == b2 && z1 == z2,
             _ => false,
         };
         if !compat {