fix(attestation): use is_multiple_of for checkpoint interval

Raising rust-version to 1.89 enables clippy's manual_is_multiple_of
lint (MSRV-gated at 1.87), which fails `-D warnings` on the evidence
checkpoint check. CHECKPOINT_INTERVAL is a non-zero const, so
is_multiple_of is behaviour-identical to the `% == 0` it replaces.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PFaqv6iBXrt3W5Akd1Fq5k

Author: claude

src/attestation/evidence.rs

@@ -142,7 +142,7 @@ impl EvidenceAccumulator {
         }
 
         // Emit checkpoint every CHECKPOINT_INTERVAL files
-        if self.files_read % CHECKPOINT_INTERVAL == 0 {
+        if self.files_read.is_multiple_of(CHECKPOINT_INTERVAL) {
             let snapshot = self.rolling_hasher.clone().finalize();
             self.checkpoints.push(EvidenceCheckpoint {
                 files_at_checkpoint: self.files_read,