chore(msrv): pin rust-version = "1.85"; add MSRV lane to rust-ci (#76)

Closes #57.

Cargo.toml uses `edition = "2024"` which stabilised in Rust 1.85, but
no MSRV was recorded. Pin `rust-version = "1.85"` and add a CI matrix
lane on that exact toolchain alongside `stable`, so MSRV breakage shows
up as a check failure instead of as a downstream user surprise.

The check job (clippy + fmt) stays on `stable` only — those tools are
allowed to evolve faster than the MSRV gate.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/rust-ci.yml

@@ -42,28 +42,38 @@ jobs:
         run: cargo clippy --all-targets -- -D warnings
 
   test:
-    name: Cargo test
+    name: Cargo test (${{ matrix.rust }})
     runs-on: ubuntu-latest
     needs: check
     if: hashFiles('Cargo.toml') != ''
+    strategy:
+      fail-fast: false
+      matrix:
+        # `1.85` matches Cargo.toml `rust-version` — the MSRV gate.
+        # `stable` keeps the lane honest against the latest stable.
+        rust: ['1.85', 'stable']
 
     steps:
       - name: Checkout repository
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
       - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@4be9e76fd7c4901c61fb841f559994984270fce7 # stable
+        uses: dtolnay/rust-toolchain@4be9e76fd7c4901c61fb841f559994984270fce7 # pinned
+        with:
+          toolchain: ${{ matrix.rust }}
 
       - name: Cache cargo registry and build
         uses: Swatinem/rust-cache@779680da715d629ac1d338a641029a2f4372abb5 # v2
+        with:
+          key: ${{ matrix.rust }}
 
       - name: Run tests
         run: cargo test --all-targets
 
       - name: Write summary
         if: always()
         run: |
-          echo "## Rust CI Results" >> "$GITHUB_STEP_SUMMARY"
+          echo "## Rust CI Results (${{ matrix.rust }})" >> "$GITHUB_STEP_SUMMARY"
           echo "" >> "$GITHUB_STEP_SUMMARY"
           echo "- **cargo check**: passed" >> "$GITHUB_STEP_SUMMARY"
           echo "- **cargo test**: completed" >> "$GITHUB_STEP_SUMMARY"

Cargo.toml

@@ -3,6 +3,7 @@
 name = "verisimiser"
 version = "0.1.0"
 edition = "2024"
+rust-version = "1.85"
 authors = ["Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>"]
 description = "Augment any database with VeriSimDB octad capabilities — drift detection, provenance, temporal versioning, and modality overlays"
 license-file = "LICENSE"