test: add failing tests for cache_max_bytes config and cache eviction

Red phase for #51: covers strict cache_max_bytes parsing (invalid
explicit values abort naming key and value), the computed default of
max(75% of free space, 500 MiB) via an injectable free-space probe,
explicit-value-no-floor, zero-disables-cache, size accounting over
source blobs and variants, LRU eviction under the limit, the
multi-referenced blob case, write-pressure and periodic eviction
triggers, and startup reconciliation. Minimal API skeletons keep the
tree compiling and lint-clean; only the new tests fail.

internal/imgcache/cache.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"log/slog"
 	"path/filepath"
 	"time"
 
@@ -27,6 +28,16 @@ type CacheConfig struct {
 	StateDir    string
 	CacheTTL    time.Duration
 	NegativeTTL time.Duration
+
+	// MaxBytes is the disk cache size limit in bytes. Zero disables
+	// the disk cache entirely (no reads, no writes, no eviction); the
+	// config layer supplies the computed default when the operator
+	// omits cache_max_bytes.
+	MaxBytes int64
+
+	// Logger receives accounting and eviction log output. A nil
+	// Logger means slog.Default().
+	Logger *slog.Logger
 }
 
 // variantMeta stores content type for fast cache hits without reading .meta file.