Defer to native tracking on 1.11+, make @public expansion pure

[AntonOresten]

Summary

• No fallback storage on 1.11+. The hidden per-module Symbol("#Republic_public_names") const was being written on every version but only ever read pre-1.11 (ispublic/public_names defer to Base on 1.11+). It is now injected on pre-1.11 only — on modern Julia the native publicp binding flag is the sole tracking and Republic adds no binding to consumer modules. The Base.invokelatest workaround in _get_storage is dropped: storage access is now confined to pre-world-partitioned-bindings Julia, where a plain getfield on a just-eval'd const is safe.
• @public expansion is pure. The macro previously ran Core.eval and mutated storage during expansion, so @macroexpand @public foo (or any tool that expands without evaluating) mutated the module, and the export-conflict check fired at expansion time. It now emits a _check_not_exported call followed by either the native Expr(:public, ...) (1.11+) or a _track_public call (pre-1.11). The conflict error consequently fires at statement execution and is a plain ErrorException rather than a LoadError.
• Uniform 1.12 conflict semantics. Running the suite on 1.11 revealed that the native cannot declare X public; it is already declared exported error is a 1.12 addition — 1.11's native keyword silently tolerates public-after-export (the old test only passed there because the expansion-time check threw first). @public now emits its own check on every version, so 1.6–1.11 match 1.12+ behavior.
• New public reflection helper has_native_public() — the single home of the v"1.11.0-DEV.469" gate, replacing the literal scattered across src and tests. Exported via @public and documented in the README.

Test plan

• Storage testsets now assert the hidden binding is absent on 1.11+ and present pre-1.11.
• New testset: @macroexpand @public foo leaves no trace (no storage, nothing marked public).
• Conflict-error testset updated to accept the execution-time ErrorException (unwrapping LoadError where applicable).
• Full suite run locally on 1.6.7, 1.10.11, 1.11.9, and 1.12.6 — all pass.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (96c541f) to head (2dec196).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines          274       277    +3     
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+ Hits           274       277    +3     

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