feat: Add file loading code for flag overrides

[kinyoklion]

Summary

This is the first of a series that adds flag overrides, as defined by the OVERRIDE specification, to the Ruby server SDK. The existing FDv1 and FDv2 file data sources keep their current behavior: this PR does not change lib/ldclient-rb/impl/integrations/file_data_source.rb, file_data_source_v2.rb, or their specs, and the override feature is additive throughout the series.

The file-based override source must tolerate a missing file, a file that is being written, and a burst of change notifications, and it must keep the last good data when a reload fails. This change adds LaunchDarkly::Impl::FileData, the file reading, parsing, merging, and reloading code that source needs:

• Document parsing for JSON and YAML. The parser checks that the document and its flags, flagValues, and segments members are objects.
• An ordered merge of several documents with duplicate keys handling of fail or ignore, expansion of each flagValues entry into a flag that is off and serves the value (so an evaluation reports the OFF reason kind, as the specification describes), deserialization into the data model classes, and per-document entry counts.
• A reloader that serializes reloads, debounces change signals, keeps the last good result when a reload fails, retries after a bounded delay, reports each distinct failure once, and can skip a reload whose content is byte-identical to the last applied content. A configured file that does not exist can be treated as a file with no content.
• A stat-based poller that compares existence, modification time, and size. A file that appears or disappears is a change.
• A watcher that uses rb-inotify on Linux (a dependency of the optional listen gem) to watch the directory of each file without descending into subdirectories, and listen elsewhere. It picks up a file that does not exist yet when it appears, and it retries once per second when a directory does not exist. Non-recursive watching matters because listen scans the whole tree under a watched directory and fails when a subdirectory cannot be read, as happens for a file in the system temporary directory.

A new spec, spec/integrations/file_data_source_compatibility_spec.rb, pins the behaviors of the existing file data sources that their other specs did not assert (per-file version numbering, keying by the entry's own key, the on-plus-fallthrough expansion of flagValues, no retry of a failed load, log messages, modification-time-only polling, no reaction to a deleted file when polling, the FDv1 poller's reload cadence after a change, and use of listen when it is present), so that they stay as they are while this code exists beside them.

Verification: unit specs for the document parser, merge, poller, reloader (including debounce, retry, last-good retention, sequential reloads, and stop), and watcher (including a directory with an unreadable subdirectory and the inotify thread lifecycle), the new compatibility pins, and the unchanged existing file data source specs. Full suite and RuboCop are clean. Each new spec was checked against a deliberate defect in the code it covers, and each compatibility pin against a deliberate change to the existing source it pins.