feat(internal): add stat-based file change poller

[kinyoklion]

Adds a Poller to internal/filedata that detects file changes by examining modification time and size on a fixed interval. It exists for environments where file system notifications are unavailable or unreliable, such as network mounts, some containers, and Kubernetes ConfigMap symlink swaps. The file-based override source offers it as an alternative or a supplement to watching, and the file data sources can adopt it later.

A file that appears or disappears counts as a change. A file that os.Stat cannot examine counts as absent. The poller compares only modification time and size, so a rewrite that keeps both values is not detected. Detection is otherwise generous. The callback feeds a Reloader.Trigger, whose debouncing and skip-unchanged handling absorb spurious detections, so the poller itself stays small.

Close signals the poll loop and returns without waiting for an examination or callback in progress, matching the Reloader. A file system that does not respond must not block shutdown, so the callback can run once more shortly after Close returns. Consumers tolerate that as they do a late reload.

Internal only. Nothing in this PR consumes it.

SDK-2654

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Note

Overview
Introduces a new Poller in internal/filedata that watches a list of paths on a fixed interval and invokes an onChange callback when os.Stat reports a different existence, modification time, or size. It is meant for setups where fsnotify-style watching is unreliable (e.g. network mounts, ConfigMap symlink swaps), with the intent to drive Reloader.Trigger later; nothing in this PR wires it up yet.

NewPoller snapshots state before returning so the first tick does not fire spuriously; Close stops the loop without waiting on in-flight stats or callbacks (same shutdown posture as Reloader), with tests covering multi-file watches, create/delete, and non-blocking close while a callback blocks.

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