chore(acir): enforce side-effects predicate scope

[AztecBot]

Summary

• hide the ambient ACIR side-effects latch behind a private SideEffectsLatch newtype and require lowering code to choose a named accessor
• track whether the current instruction declares requires_acir_gen_predicate in debug builds and assert on every scoped predicate read
• move the safe-index disabled-array guard before predicate acquisition, and stop forwarding the latch to black-box intrinsics which do not consume it
• retain an explicit, closed escape hatch for the one fail-safe stale read used only to skip unnecessary gating

The tracking field and assertion are absent from release builds; the newtype/accessors do not change circuit generation.

Refs noir-lang/noir-claude#1654.
Refs https://gist.github.com/AztecBot/ab38996b864f2a2bbabbd456708d72c6#8-suggested-sequencing

Testing

• cargo test -p noirc_evaluator predicate_rejects_an_out_of_scope_read
• cargo test -p noirc_evaluator index_read_under_disabled_predicate
• cargo test -p noirc_evaluator --lib (1,947 passed; 15 ignored)
• cargo check -p noirc_evaluator --release
• NOIR_AST_FUZZER_BUDGET_SECS=3 cargo test -p noir_ast_fuzzer_fuzz acir_vs_brillig -- --nocapture

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Created by claudebox · group: slackbot · requested by Tom (@TomAFrench) · Slack thread