test: add contentLock, a per-key mutex for content-hash exclusion

Introduces the keyed exclusion primitive that StoreSource and
evictSourceBlob will hold across their full operation, so a store and
an eviction racing on identical content bytes cannot interleave.
Covered here in isolation: same-key exclusion, independence across
distinct keys, and that the entry map does not grow unbounded.

internal/imgcache/contentlock.go

@@ -0,0 +1,64 @@
+package imgcache
+
+import "sync"
+
+// contentLock provides per-key mutual exclusion for content-hash keyed
+// operations. StoreSource and evictSourceBlob each hold a content
+// hash's lock for the full duration of their file-plus-accounting-row
+// work, so a store and an eviction racing on identical content bytes
+// (the real SHA-256 content-addressed dedup case, not a contrived one)
+// can never interleave: the unlink of an evicted blob's file can never
+// race the creation of a fresh database row for a concurrently
+// re-stored copy of the same content. Entries are removed once no
+// goroutine holds or is waiting for them, so a long-running process
+// does not accumulate memory proportional to the number of distinct
+// content hashes it has ever seen.
+type contentLock struct {
+	mu      sync.Mutex
+	entries map[string]*contentLockEntry
+}
+
+// contentLockEntry is one key's exclusion lock plus a count of
+// goroutines currently holding or waiting to acquire it, used to know
+// when it is safe to remove the entry from the map.
+type contentLockEntry struct {
+	mu    sync.Mutex
+	count int
+}
+
+// newContentLock creates an empty contentLock.
+func newContentLock() *contentLock {
+	return &contentLock{entries: make(map[string]*contentLockEntry)}
+}
+
+// Lock acquires exclusive access for key, blocking until it is
+// available, and returns a function that releases it. The caller must
+// invoke the returned function exactly once to release the lock.
+func (c *contentLock) Lock(key string) func() {
+	c.mu.Lock()
+
+	entry, ok := c.entries[key]
+	if !ok {
+		entry = &contentLockEntry{}
+		c.entries[key] = entry
+	}
+
+	entry.count++
+
+	c.mu.Unlock()
+
+	entry.mu.Lock()
+
+	return func() {
+		entry.mu.Unlock()
+
+		c.mu.Lock()
+
+		entry.count--
+		if entry.count == 0 {
+			delete(c.entries, key)
+		}
+
+		c.mu.Unlock()
+	}
+}