Test that the default cache_max_bytes counts what the cache holds

With a fake free-space probe, an empty cache with 4 GiB free gets a
3 GiB default, and the same cache once it holds those 3 GiB, with
1 GiB left free, must keep 3 GiB. The default is still 75% of the
free space alone, so the second check fails: this is the bug in
#184. computeDefaultMaxBytes
only calls the config's existing computation so the test compiles.

Model: opus-5-5

internal/imgcache/cachesize.go

@@ -0,0 +1,17 @@
+package imgcache
+
+import (
+	"context"
+	"path/filepath"
+
+	"sneak.berlin/go/pixa/internal/config"
+)
+
+// computeDefaultMaxBytes returns the default cache size limit for an
+// omitted cache_max_bytes, for the filesystem holding <state_dir>/cache/.
+func (c *Cache) computeDefaultMaxBytes(
+	_ context.Context, probe config.FreeSpaceProbeFunc,
+) (int64, error) {
+	return config.ComputeDefaultCacheMaxBytes(
+		filepath.Join(c.config.StateDir, "cache"), probe)
+}