feat: serve flag overrides while uninitialized and mark override evaluations in events

[kinyoklion]

Completes the client-side behavior of the override system:

• Not-initialized short-circuit: when the client has no data from LaunchDarkly, the per-key evaluation gate now consults the override layer before returning the caller's default. An overridden flag falls through to the store read, which serves it via the overlay — including prerequisite and segment resolution against the merged view. Non-overridden flags short-circuit exactly as before, and removing an override restores the short-circuit on the next evaluation. The check only executes on the not-Refreshed branch, so the initialized hot path is untouched.
• AllFlagsState: when uninitialized but the override layer has entries, the state is built from the merged view — a valid state containing only the overridden flags — instead of invalid-empty. This mirrors the existing behavior of serving known-incomplete cached data as valid, and keeps overrides useful for bootstrap endpoints during an outage. With no overrides, the invalid-empty behavior is unchanged. (The spec doesn't prescribe this case; flagged as a decision.)
• Events: the flag's override marker rides into FlagEventProperties for both top-level and prerequisite evaluations, which is what drives the events pipeline's separate override summary counters and its suppression of individual feature/debug events (feat: mark override evaluations in summary counters and suppress their individual events go-sdk-events#54).

Client-level tests cover the gate matrix (overridden vs. not, with removal), the AllFlagsState three-way branch, the event marker, the marked evaluation reason end-to-end, and flag-change notifications firing when overrides change on a running client.

go.mod points at pseudo-versions of the in-flight go-server-sdk-evaluation and go-sdk-events branches; those get pinned to real releases at the end of the series.

Based on #409.

SDK-2655