feat: add file-based override source (ldoverrides)

[kinyoklion]

Reopens #412, which GitHub auto-marked as merged when its branch was merged into the old integration branch it was based on. The branch is rebuilt as clean cherry-picks onto rlamb/overrides-base, the junction of the two prerequisite stacks (file machinery #405–#407, override system #404/#408–#411); the diff is identical in content to #412 plus its follow-up lifecycle comment. Review discussion lives on #412.

Adds the public ldoverrides package with FileSource(), the file-based override source defined by the OVERRIDE spec, completing the user-facing feature:

config := ld.Config{
    DataSystem: ldcomponents.DataSystem().Default().
        Overrides(ldoverrides.FileSource().FilePaths("/etc/ld/overrides.json")),
}

• Same document format as the file data sources (flags, flagValues, segments; JSON and YAML), with flagValues entries expanded into full flag definitions. Multiple files merge in configured order under the configured duplicate-key handling.
• Built on the shared internal/filedata machinery: every reload is serialized, debounced, all-or-nothing with last-good retention, retried after failures, and skipped when contents are unchanged. Missing files at startup just mean no overrides until they appear.
• Watching by default (ldfilewatch/fsnotify), with an optional stat-based Poll mode for filesystems where notifications are unreliable; both may run together. Poll interval defaults to one second, clamped to a one-second minimum.

Package tests cover initial load, YAML, multi-file ordering and duplicates, missing-file startup and recovery, watch and poll reload modes, malformed-edit retention and recovery, interval clamping, and close semantics; a client-level test drives hot add/change/remove end-to-end on a running uninitialized client.

SDK-2656