refactor(node): move the subtitle handler out of webrtc_server

SubtitlesMixin in transport/webrtc/apps/subtitles.py; _locate, which it
shares with the files, music and streaming handlers, in webrtc/disk.py.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc/disk.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc/disk.py
new file mode 100644
index 0000000..fd3184c
--- /dev/null
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/disk.py
@@ -0,0 +1,23 @@
+"""Blocking disk work the session hands to `off_disk`."""
+
+from pathlib import Path
+
+from meshbay_node.roots import ROOT_NOT_SERVED, RootSet, entry_abs_path
+
+
+def _locate(roots: RootSet, entry) -> tuple[Path | None, str | None]:
+    """
+    Where an entry is, and whether it is readable — or the refusal to send.
+
+    Both halves are syscalls: `resolve()` walks the path and `exists()` stats
+    it, and a stat is what *wakes* a sleeping disk. Leaving either on the event
+    loop and offloading only the read would move the stall rather than remove
+    it, and the read would then find the disk already awake. Blocking; called
+    through `off_disk`.
+    """
+    path = entry_abs_path(roots, entry)
+    if path is None:
+        return None, ROOT_NOT_SERVED
+    if not path.exists():
+        return None, "File not on disk"
+    return path, None