feat: add file-based override source (ldoverrides)

[kinyoklion]

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, so every reload is serialized, debounced, all-or-nothing with last-good retention, retried after failures, and skipped when file contents are unchanged. A malformed or mid-write edit can never empty the override layer or stall the source; missing files at startup just mean no overrides until they appear.
• Change detection is watching by default (ldfilewatch/fsnotify, which already handles rename-replace editors by watching the parent directory), with an optional stat-based Poll mode for filesystems where notifications are unreliable — network mounts, some containers, Kubernetes ConfigMap symlink swaps. Both may run together; the poll interval defaults to one second and is clamped to a one-second minimum.
• The package is named for the feature rather than the mechanism so anticipated future sources (HTTP, object storage) have a home.

Package tests cover initial load, YAML, multi-file ordering and duplicate handling, 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 the whole loop end-to-end: an uninitialized client picks up an operator's file edits — add, change, remove — with evaluations, the isOverride reason, and the restored short-circuit following along, no restart involved.

This PR targets the rlamb/overrides integration branch because it is the junction of the two in-flight stacks: the internal/filedata reload machinery (#405–#407) and the override system (#404, #408–#411).

SDK-2656