fix(ci): split Miri into parallel jobs to fix 25m timeout

The single Miri job ran core + sparse sequentially, consistently
exceeding the 25-minute timeout. Split into miri-core and miri-sparse
jobs that run in parallel with 20m timeout each.

Zero test coverage regression — identical skip filters, env vars,
and feature flags preserved.

Hostile review: APPROVED (0C/0M/1m, m1 fixed)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: matte1782

.github/workflows/ci.yml

@@ -3,6 +3,7 @@
 # W18.2: CI Hardening — Documented timeouts and environment variables
 # W45.6: CI Optimization — Added Rust dependency caching, Miri skip filters
 # W46.2: Expanded Miri skips — hybrid::, quantization::product::, hnsw:: (all pure-safe, no UB risk)
+# W48.0: Split Miri into parallel jobs (core + sparse) to fix 25m timeout
 #
 # Local Simulation:
 #   cargo xtask ci-check
@@ -17,7 +18,8 @@
 #   lint:       5 minutes  (cargo fmt + clippy)
 #   wasm-check: 5 minutes  (cargo check --target wasm32-unknown-unknown)
 #   fuzz-check: 5 minutes  (cargo +nightly check)
-#   miri:       25 minutes (cargo +nightly miri test, focused on unsafe-adjacent code only)
+#   miri-core:   20 minutes (cargo +nightly miri test, core features)
+#   miri-sparse: 20 minutes (cargo +nightly miri test, sparse feature)
 
 name: EdgeVec CI
 
@@ -113,7 +115,7 @@ jobs:
       - name: Check ALL Fuzz Targets Compile
         run: cargo +nightly check --manifest-path fuzz/Cargo.toml --all-targets
 
-  # Job 5: Miri Safety Audit (UB detection)
+  # Job 5a: Miri Safety Audit — Core (UB detection)
   # Focuses ONLY on unsafe-adjacent code: SIMD, persistence, storage binary ops
   # Skipped modules (pure-safe, no unsafe, or too slow under Miri):
   #   - filter::, metadata:: — zero unsafe code
@@ -122,10 +124,10 @@ jobs:
   #   - hnsw:: — graph search is safe-only, slow under Miri instrumentation
   #   - wasm — not available in Miri
   #   - test_bq_vs_f32_recall_comparison — slow integration test
-  miri:
-    name: Miri Safety Check
+  miri-core:
+    name: Miri Safety Check (core)
     runs-on: ubuntu-latest
-    timeout-minutes: 25
+    timeout-minutes: 20
     steps:
       - uses: actions/checkout@v4
 
@@ -153,6 +155,23 @@ jobs:
           MIRIFLAGS: "-Zmiri-disable-isolation -Zmiri-symbolic-alignment-check -Zmiri-permissive-provenance"
           PROPTEST_CASES: "4"
 
+  # Job 5b: Miri Safety Audit — Sparse feature (runs in parallel with 5a)
+  # Skip rationale: see Job 5a comment block above
+  miri-sparse:
+    name: Miri Safety Check (sparse)
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Nightly Rust with Miri
+        uses: dtolnay/rust-toolchain@nightly
+        with:
+          components: miri
+
+      - name: Cache Rust dependencies
+        uses: Swatinem/rust-cache@v2
+
       - name: Run Miri (sparse feature)
         run: |
           cargo +nightly miri test --lib --no-default-features --features sparse -- \