fix: close the remaining review findings (cross-process lock, partials)

Second Codex (gpt-6.1-sol, xhigh) pass over 60634d3, each item verified
before fixing:

- new: the sync recheck hashed an existing empty database before schema
  setup, so the first sync falsely reported an external change. Hash
  after setup; a published snapshot already has the schema and keeps
  its bytes.
- sync, import, and entity edits now hold a cross-process lock
  (<db>.lock, std File::lock), so a sync cannot replace the database
  under an import running in another process. Import validates and
  downloads before taking the lock, keeping its error order. MSRV
  1.85 -> 1.89 for File::lock.
- the row-count rebuild branch also never downgrades history.
- cache reads no longer delete corrupt entries (a concurrent writer may
  have just replaced them); the next put overwrites atomically.
- idx-mcp validates the changes limit before querying and runs each
  requested section or analysis once.
- XLSX parsing bounds total cell text, so one large shared string
  referenced from many cells cannot exhaust memory.
- PDF/archive percentages must be 0-100%; HHI uses saturating math.

CLI goldens unchanged. Live A/B vs 0.2.4: 16 passed, discover hits the
IDX 403 on both builds (exit 4 vs 1, intended).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/idx-core/src/ownership/db.rs

@@ -1196,7 +1196,10 @@ pub fn compute_concentration(percentages_bps: &[i64]) -> ConcentrationMetrics {
     let top1_bps = values.first().copied().unwrap_or(0);
     let top3_bps: i64 = values.iter().take(3).sum();
     let total_bps: i64 = values.iter().sum();
-    let hhi: i64 = values.iter().map(|p| (p * p) / 10000).sum();
+    let hhi: i64 = values
+        .iter()
+        .map(|p| p.saturating_mul(*p) / 10000)
+        .fold(0, i64::saturating_add);
 
     ConcentrationMetrics {
         top1_bps,