Support `$.flags` context values in segment conditions

Dependent flags let a segment condition read another flag's result via a `$.flags..` property, so that flag has to be resolved before the condition is evaluated. Behaviour is defined by the shared cases in Flagsmith/engine-test-data#59; schema in Flagsmith/flagsmith#8396; reference implementation in Flagsmith/flagsmith-engine#343.

The reference implementation resolves a flag lazily, on first read, rather than scanning every condition up front to discover dependencies. Three things in this engine currently prevent that:

- `get_value_from_jsonpath` calls `serde_json::to_value(ec)` on **every** lookup, serialising the whole context (~268 µs on a 263-feature environment, of which the query itself is 389 ns). A lazily resolved `flags` map has nothing to be inserted into, and lookups already dominate evaluation.
- `JsonPath::parse` also runs on every lookup (~1.5 µs), uncached.
- `serde_json::Value` exposes no lookup hook, so resolution must be triggered by inspecting the parsed path. `serde_json_path::JsonPath` keeps its `Query` private, though the AST is public in `serde_json_path_core::spec`.

### Acceptance criteria

- `engine-test-data` is bumped from `v3.7.0` to the tag containing the flag dependency cases, and they pass.
- An environment with no `$.flags` conditions gains no per-evaluation cost.
- Dependency detection uses the parser's AST, not a bespoke matcher, so all pinned spellings of a query agree.

Start by locating the engine's get_value_from_jsonpath path and the JsonPath::parse call, then read the evaluation flow around segment conditions and the serde_json_path_core::spec AST. Use the shared dependency cases from engine-test-data and compare the reference implementation in flagsmith-engine#343. Done means the dependency cases pass, contexts without $.flags avoid per-evaluation overhead, and detection follows the parser AST.

Written by the indexing model from the issue text.