fix: fetch flags only when storage holds another identity's flags

[Zaimwa9]

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Changes

Follow-up to #94. Based on feat/experimentation; retarget to main once that merges.

The client now remembers which identity the stored flags were fetched for (null for environment flags, reset() and a fresh process). Lookups that take a user fetch from the API only when that identity differs from the stored one, and read storage otherwise. An explicit reload still wins.

• getExperimentFlag(user:) no longer hits /identities on every call; same identity as the last fetch is served from storage, a different identity is fetched. Exposures keep the guarantee from feat: surface experiment metadata on flags and add event tracking #94: never attributed against another identity's assignment.
• hasFeatureFlag, isFeatureFlagEnabled and getFeatureFlagValue share the same rule through _getFlagByName. Previously a user different from the last fetch silently returned the previous identity's flags from storage; now it fetches once. Same identity or no user: unchanged, no request.

Behaviour change to the three existing methods only on the path that returned the wrong identity's flags.

How did you test this code?

flutter analyze clean, flutter test 165 passing. New cases: same identity makes no request; environment flags overwriting storage trigger a fetch for the cached identity; hasFeatureFlag with a switched identity fetches once and then reads storage.

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