feat: Add shared file data code with reload, retry, and polling

[kinyoklion]

Summary

Adds ldclient.impl.integrations.files.filedata, the file reading, parsing, and merging logic shared by the components that load flag and segment data from local files. This is groundwork for the file-based override source described by the OVERRIDE specification, which needs the same file handling that the specification requires: ordered multi-file merging with duplicate keys handling, a reload that keeps the last good data across a malformed edit and retries, debouncing of change notifications, a polling change detector as well as a watching one, and correct handling of a configured file that does not exist yet.

What the module provides:

• Document parsing. A document that starts with an opening brace is parsed as JSON and any other document as YAML, so JSON files no longer depend on the YAML parser being installed. Flag and segment definitions are decoded into the model classes while the file is read, so an invalid definition fails that load instead of failing later inside the store. A definition may still omit its own key and version, which are filled in from the map key and a version of 1.
• Merging. Files are combined in the configured order. The duplicate keys handling is fail (the load fails) or ignore (the first file's entry is kept). The result records how many entries each file supplied.
• Reloader. Serializes reloads, debounces change signals with a settle window that each signal extends, keeps the last good data by not applying a failed load, retries a failed load after a bounded delay so a file observed mid-write recovers without another notification, reports an identical failure once, skips an application whose file contents are byte-identical to the last applied contents, and applies a success after a failure even when the contents did not change. It can treat a configured file that does not exist as a file with no content. Closing does not wait for an in-flight reload.
• Poller. Compares modification time and size on an interval, so a same-size rewrite and a same-time size change are both detected, as are files that appear or disappear.
• Watcher. Uses the watchdog package. It watches the directory of each file so an absent file is picked up when it appears, matches the destination of a move event so a file written by rename is detected, retries a directory that does not exist yet, and reacts only to notifications that can change a file's content or presence. The watchdog inotify mask also reports a file being opened or read, and a reload reads the files, so without that filter every reload would trigger the next one.

Both existing file data sources are rebuilt on this module. Their files must still exist and duplicate keys still fail the load. Two behaviors change:

• The FDv2 file synchronizer keeps the last good data across a failed load, reports an interrupted state, and retries. Before, a failed initial load ended the synchronizer with an off state, and a later failure had no retry.
• A flagValues entry expands into an off flag that serves its single variation as the off variation. This is the form the Go SDK has always used, and it is the form the OVERRIDE specification describes for a value-only override. The evaluation reason kind for such a flag is OFF where it was FALLTHROUGH. The value is unchanged.

Tests cover parsing, merging, loading, the reloader (initial load, failure retention, debounce coalescing and window extension, retry with and without further signals, identical-failure reporting, skip-unchanged and recovery, serialization of concurrent reloads, close semantics, worker thread lifecycle), the poller, and the watcher.