feat: Support $.flags context values in segment conditions

[khvn26]

Closes #245.

Changes

• Environment flags evaluate segments. get_environment_flags() in local evaluation / offline mode no longer strips segments from the evaluation context (fix: get_environment_flags includes segments in evaluation context #179 added the stripping), so identity-less $.flags.<feature>.<enabled|value|variant> and $.environment.* conditions now apply to environment flags.
• Lazy identity flags resolve $.flags dependencies. Flags.get_flag() evaluates against a trimmed context: just the queried feature and the segments that override it. That trimmed context leaves out the flags a $.flags condition reads, so those conditions never matched. Features overridden by a segment with a $.flags condition are now found once per environment refresh (build_flag_dependent_feature_names). For those features, get_flag() evaluates the full context and memoises every resulting flag. All other features keep the trimmed fast path.
  • Detection parses each condition property with jsonpath-rfc9535, the parser the engine uses, and follows the engine's rules: only $.-prefixed properties that compile are JSONPath. A property is treated as reading $.flags unless its first segment selects top-level keys by name and none of them is flags. Wildcards, filters and $.. count as possibly reading it, so a wrong guess only costs speed, never correctness. jsonpath-rfc9535 is now a direct dependency, with the same >=0.1.5,<1 range as the engine.
• The engine dependency is already on ^11.1.1 (deps: update dependency flagsmith-flag-engine to v11.1.0 #252, deps: bump flagsmith-engine to 11.1.1 #259), which includes $.flags support (feat: Evaluate flags that depend on other flags flagsmith-engine#343).

Behaviour change

Environment flags can now pick up segment overrides from segments whose rules match without an identity, e.g. $.environment.name conditions or IS_NOT_SET on a trait. This is what #245 asks for, and it matches the get_evaluation_result semantics discussed in Flagsmith/flagsmith#6051. Note that remote /api/v1/flags may still return plain environment defaults.

Tests

• test_get_environment_flags_omits_segments_from_evaluation_context becomes ..._includes_segments_....
• End-to-end tests for $.flags / $.environment overrides on environment flags and identity flags. These include a dependency that is itself overridden for an identity.
• Unit tests for $.flags property detection (dot/bracket/descendant forms, nested rules) and for full-context resolution and memoisation in lazy Flags.