feat(filer): add atime to FuseAttributes + TouchAccessTime RPC (#9556)

* feat(filer): add atime field and TouchAccessTime RPC to filer proto

Introduce POSIX-style access-time tracking on the filer:
- FuseAttributes gains atime (field 22) and atime_ns (field 23).
- New TouchAccessTime RPC (and Touch{Access,Time}{Request,Response})
  lets read paths bump atime without going through UpdateEntry's
  chunk-rewrite/EqualEntry short-circuit.

Additive proto changes only; zero atime is treated as unset and
existing clients are unaffected. Java client proto is kept in lock
step.

Co-authored-by: Cursor <cursoragent@cursor.com>

* feat(filer): wire Atime through Attr codec with mtime fallback

Add Attr.Atime and round-trip it through EntryAttributeToPb /
EntryAttributeToExistingPb / PbToEntryAttribute. A zero proto atime
decodes as Mtime, so legacy entries report a sensible value and
freshly-created/updated entries default Atime to Mtime when callers
do not set it explicitly.

CreateEntry and UpdateEntry stamp Atime = Mtime (or Crtime) when it
is zero. TouchAccessTime later bypasses this path to write atime
alone via Store.UpdateEntry.

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(filer): preserve atime in first epoch second on decode

The Atime decode branch previously treated any attr.Atime == 0 as
unset and overwrote it with Mtime, which drops valid timestamps in
the first second of the unix epoch where attr.Atime is 0 but
attr.AtimeNs > 0. Check both fields so we only fall back to Mtime
when both are zero.

Co-authored-by: Cursor <cursoragent@cursor.com>

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

weed/filer/entry_codec.go

@@ -85,6 +85,8 @@ func EntryAttributeToPb(entry *Entry) *filer_pb.FuseAttributes {
 		MtimeNs:       int32(entry.Attr.Mtime.Nanosecond()),
 		Ctime:         entry.Attr.Ctime.Unix(),
 		CtimeNs:       int32(entry.Attr.Ctime.Nanosecond()),
+		Atime:         atimeSecondsForPb(entry.Attr),
+		AtimeNs:       atimeNanosForPb(entry.Attr),
 		FileMode:      uint32(entry.Attr.Mode),
 		Uid:           entry.Uid,
 		Gid:           entry.Gid,
@@ -100,6 +102,20 @@ func EntryAttributeToPb(entry *Entry) *filer_pb.FuseAttributes {
 	}
 }
 
+func atimeSecondsForPb(attr Attr) int64 {
+	if attr.Atime.IsZero() {
+		return 0
+	}
+	return attr.Atime.Unix()
+}
+
+func atimeNanosForPb(attr Attr) int32 {
+	if attr.Atime.IsZero() {
+		return 0
+	}
+	return int32(attr.Atime.Nanosecond())
+}
+
 // EntryAttributeToExistingPb fills an existing FuseAttributes to avoid allocation.
 // Safe to call with nil attr (will return early without populating).
 func EntryAttributeToExistingPb(entry *Entry, attr *filer_pb.FuseAttributes) {
@@ -111,6 +127,8 @@ func EntryAttributeToExistingPb(entry *Entry, attr *filer_pb.FuseAttributes) {
 	attr.MtimeNs = int32(entry.Attr.Mtime.Nanosecond())
 	attr.Ctime = entry.Attr.Ctime.Unix()
 	attr.CtimeNs = int32(entry.Attr.Ctime.Nanosecond())
+	attr.Atime = atimeSecondsForPb(entry.Attr)
+	attr.AtimeNs = atimeNanosForPb(entry.Attr)
 	attr.FileMode = uint32(entry.Attr.Mode)
 	attr.Uid = entry.Uid
 	attr.Gid = entry.Gid
@@ -140,6 +158,11 @@ func PbToEntryAttribute(attr *filer_pb.FuseAttributes) Attr {
 	} else {
 		t.Ctime = t.Mtime
 	}
+	if attr.Atime != 0 || attr.AtimeNs != 0 {
+		t.Atime = time.Unix(attr.Atime, int64(attr.AtimeNs))
+	} else {
+		t.Atime = t.Mtime
+	}
 	t.Mode = os.FileMode(attr.FileMode)
 	t.Uid = attr.Uid
 	t.Gid = attr.Gid