fix(mount): bound reader cache memory across open files (#11220)

* fix(filer): bound retained reader cache buffers by bytes

* test(filer): keep in-flight downloads during cache trimming

* feat(mem): expose pooled allocation capacity for byte reservations

* fix(mount): share a configurable reader buffer budget across files

* fix(filer): release failed prefetch slots and memory reservations

* feat(mount): expose a soft Go runtime memory limit

* docs(filer): restore shared-download rationale in startCaching

The one-line comment replacing the original context.Background() explanation was too thin for readChunkAt to cross-reference shared resource semantics. Restore a concise note on why request cancellation must not abort a download shared by concurrent readers.

* test(filer): loosen reader cache test deadlines to 5s

Three tests used 1-second deadlines that can flake on CI under load:
TestReaderCacheBudgetInFlight, TestReaderCacheEvictionDoesNotHoldCacheLock,
and TestReaderCacheFailedPrefetchReleasesBudget. Increase to 5 seconds.

* test(filer): cover re-read after reader cache eviction

Add TestReaderCacheReReadAfterEviction: reads chunk 'a', reads chunk 'b'
(evicting 'a' via budget pressure), then re-reads 'a' and asserts a
fresh download returns correct data. Verifies the core correctness
property that eviction never exposes missing or stale data to readers.

weed/command/mount_memory_test.go

@@ -0,0 +1,39 @@
+//go:build linux || darwin || freebsd || windows
+
+package command
+
+import (
+	"math"
+	"runtime/debug"
+	"testing"
+)
+
+func TestConfigureMountMemory(t *testing.T) {
+	for _, size := range []int64{-1, 0, 1, 256, math.MaxInt64 >> 20, math.MaxInt64} {
+		err := configureMountMemory(&MountOptions{readerCacheSizeMB: &size})
+		valid := size > 0 && size <= math.MaxInt64>>20
+		if (err == nil) != valid {
+			t.Errorf("size=%d: err=%v", size, err)
+		}
+	}
+}
+
+func TestConfigureMountMemoryRuntimeLimit(t *testing.T) {
+	previous := debug.SetMemoryLimit(512 << 20)
+	defer debug.SetMemoryLimit(previous)
+	for _, size := range []int64{0, -1, math.MaxInt64, 768, 0} {
+		before := debug.SetMemoryLimit(-1)
+		err := configureMountMemory(&MountOptions{memoryLimitMB: &size})
+		valid := size >= 0 && size <= math.MaxInt64>>20
+		if (err == nil) != valid {
+			t.Errorf("size=%d: err=%v", size, err)
+		}
+		want := before
+		if valid && size > 0 {
+			want = size << 20
+		}
+		if got := debug.SetMemoryLimit(-1); got != want {
+			t.Errorf("size=%d: runtime limit=%d, want %d", size, got, want)
+		}
+	}
+}