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/eviction.go

@@ -0,0 +1,41 @@
+package imgcache
+
+import (
+	"context"
+	"time"
+)
+
+// DefaultEvictionInterval is how often the background evictor checks
+// cache usage against the configured limit, in addition to the
+// write-pressure wakeups triggered by stores.
+const DefaultEvictionInterval = 5 * time.Minute
+
+// UsageBytes returns the total number of bytes of cache content
+// tracked in the database (source content blobs plus processed
+// variants). It never scans the cache directories.
+func (c *Cache) UsageBytes(_ context.Context) (int64, error) {
+	// Red phase: implementation follows the failing tests.
+	return 0, nil
+}
+
+// EvictToLimit evicts least-recently-used cache entries until total
+// tracked usage is at or below the configured MaxBytes limit. It is a
+// no-op when the cache is disabled (MaxBytes zero).
+func (c *Cache) EvictToLimit(_ context.Context) error {
+	// Red phase: implementation follows the failing tests.
+	return nil
+}
+
+// StartEviction launches the background eviction goroutine, which
+// reconciles the database accounting with the cache directories once
+// at startup and then evicts to the configured limit on the given
+// periodic interval and on write-pressure notifications.
+func (c *Cache) StartEviction(_ time.Duration) {
+	// Red phase: implementation follows the failing tests.
+}
+
+// StopEviction stops the background eviction goroutine and waits for
+// it to exit. It is safe to call when eviction was never started.
+func (c *Cache) StopEviction() {
+	// Red phase: implementation follows the failing tests.
+}