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>

other/java/client/src/main/proto/filer.proto

@@ -22,6 +22,9 @@ service SeaweedFiler {
     rpc UpdateEntry (UpdateEntryRequest) returns (UpdateEntryResponse) {
     }
 
+    rpc TouchAccessTime (TouchAccessTimeRequest) returns (TouchAccessTimeResponse) {
+    }
+
     rpc AppendToEntry (AppendToEntryRequest) returns (AppendToEntryResponse) {
     }
 
@@ -208,6 +211,8 @@ message FuseAttributes {
     int32 mtime_ns = 19; // nanosecond component of mtime (0-999999999)
     int32 ctime_ns = 20; // nanosecond component of ctime (0-999999999)
     int32 crtime_ns = 21; // nanosecond component of crtime (0-999999999)
+    int64 atime = 22; // unix time in seconds, last access time
+    int32 atime_ns = 23; // nanosecond component of atime (0-999999999)
 }
 
 message CreateEntryRequest {
@@ -247,6 +252,16 @@ message UpdateEntryResponse {
     SubscribeMetadataResponse metadata_event = 1;
 }
 
+message TouchAccessTimeRequest {
+    string directory = 1;
+    string name = 2;
+    int64 client_atime_ns = 3; // nanoseconds since epoch; filer may override with relatime
+}
+message TouchAccessTimeResponse {
+    int64 persisted_atime_ns = 1; // nanoseconds since epoch; 0 if no update was performed
+    bool updated = 2;
+}
+
 message AppendToEntryRequest {
     string directory = 1;
     string entry_name = 2;