feat(bar): surface Lex-native signals in the bar walk

[alpibrupa]

Summary

• The bar ledger's 14 items are explicitly sourced from two published books (Prompt to Production ch.16, Prompt to Evidence ch.15) — prose checklists with no concept of a typed effect system. Adding a 15th item and attributing it to either book would misrepresent its provenance.
• Wires Effect-row minimality as a CI-level probe, not just a manual review step #119's `effect_minimality` probe into `bar_check`'s `run_walk` as a separate, clearly-labeled "LEX-NATIVE SIGNALS" section instead — evidence specific to what Lex's own effect system can prove about itself, positioned between the book-sourced probes and the unprobed items.

Fixes #121

Test plan

• `lex check`/`lex fmt --check` — ok
• Full repo sweep — all pass
• `lex test` — 4/4 passed, including `test_bar.lex`
• `lex doc-sync --check` — current
• Direct tool-call verification: a driver calling `bar_check.execute({root: ".", path: "src/tools"})` shows the new section rendering correctly between the 6 probed book-sourced items and the 8 unprobed ones, with `effect_minimality`'s report text intact.

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