feat(lint): add UncheckedFlagAccess detector (#157)

[kirich1409]

Summary

• Adds UncheckedFlagAccessDetector — an Android Lint rule (WARNING, priority 8) that flags calls to @BehindFlag("flagName")-annotated functions when the call site is not inside a valid guard context
• Valid guard contexts (v1): if/when expression referencing the flag by name, or an enclosing function annotated with the same @BehindFlag
• Registers the new issue in FeaturedIssueRegistry

Implementation notes

• @BehindFlag has SOURCE retention, so annotations are read via PSI (PsiAnnotation) rather than descriptors; ConstantEvaluator.evaluate() is used to resolve string values from Kotlin sources
• USwitchClauseExpression.caseValues is checked in addition to USwitchExpression.expression to handle both when (flag) { ... } (subject form) and when { flag -> ... } (conditionless form), including lint's TestMode.IF_TO_WHEN rewrite

Test plan

• 2 positive cases (bare unguarded call; if-block with unrelated condition) → expectWarningCount(1)
• 5 negative cases: if (flag) guard, @BehindFlag enclosing function, no annotation, when (flag) subject — all expectClean()
• All 14 tests in :featured-lint-rules:test pass

Closes #157

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[kirich1409]

Closing: content merged in #176.