feat(internal): add flag/segment override layer, overlay store, and change notification diff

[kinyoklion]

Adds internal/overrides, the core of the flag-overrides feature (OVERRIDE spec): a runtime-mutable layer of flag and segment definitions that will take precedence over LaunchDarkly data at the store read boundary. Nothing consumes this package yet — FDv2 wiring and the public source API come next — so this PR is the data structures and their semantics in isolation.

• Layer: thread-safe holder of override entries, replaced wholesale on every update (an update is a full snapshot; entries absent from it are removed). Entries are stored as copies carrying the typed-model override marker, so sources stay marker-agnostic and their retained structs are never mutated; the copies are defensively re-preprocessed since the sink can't know how a source constructed them. An emptiness probe is a single atomic read, keeping a configured-but-unpopulated layer effectively free per evaluation.
• Overlay: a ReadOnlyStore that merges the layer over a base store — the override entry wins per key on Get (including over deleted-item tombstones), GetAll is the union, and initialization status delegates to the base untouched. Serving overrides from an uninitialized base works because an uninitialized memory store reports not-found rather than erroring.
• Sink: applies serialized layer replacements and computes which flags to notify. The seed set is every added, removed, or changed override entry — added and removed keys always count, because the marker alone changes what an evaluation reports, and same-key changes compare by serialized form since the layer is rebuilt from scratch each time (pointer or version comparison would notify every retained entry, or nothing). Fan-out then runs through dependency trackers built over both the old and the new merged views, because a replacement can rewire dependencies — e.g. removing a flag override restores the LaunchDarkly definition's prerequisite edges, and dependents of the override's own references only exist in the old view. A base read failure degrades fan-out but never drops the directly changed keys.

Depends on go-server-sdk-evaluation carrying the ldmodel override marker (launchdarkly/go-server-sdk-evaluation#57); go.mod points at a pseudo-version of that branch and will be pinned to the released version before merge. Based on #404 (the dependency-tracker consolidation this reuses).

SDK-2655