style: clear the last lint findings on #55's code

- goconst: extracted testVariantKeyTwo alongside testVariantKeyOne.
- paralleltest: t.Parallel() on
  TestEvictSourceBlobExcludesConcurrentStoreOfIdenticalContent. The test
  asserts that a concurrent store stays blocked for 200ms while eviction
  holds the content lock; parallel load can only make it more blocked,
  never less, so the assertion does not become timing-fragile. Verified
  over repeated full -race runs.
- gosec G115: a live, justified suppression on the int64 conversion in
  ComputeDefaultCacheMaxBytes. The preceding clamp was an explicit
  if-statement that gosec could follow; the modernize linter requires it
  to be min(), which gosec's range analysis cannot see through. The
  clamp is still there and still correct, so the conversion cannot
  overflow. Unlike the directives removed earlier in this branch, this
  one is live: nolintlint confirms it suppresses a finding that is
  actually raised.

internal/config/cachesize.go

@@ -66,6 +66,10 @@ func ComputeDefaultCacheMaxBytes(
 	computed := freeBytes / freeSpaceFractionDenominator * freeSpaceFractionNumerator
 	computed = min(computed, math.MaxInt64)
 
+	// gosec cannot see that min() above bounds computed, so it reads
+	// this conversion as potentially overflowing. It cannot: computed is
+	// at most math.MaxInt64 on every path here.
+	//nolint:gosec // G115: clamped to MaxInt64 by min above
 	limit := int64(computed)
 	limit = max(limit, DefaultCacheMaxBytesFloor)