diag: don't fire prefix-precedence hint inside parens

[danieljohnmorris]

Summary

PR #301's same-precedence prefix-pair trap hint (*/, /*, +-, -+) fires on the parenthesised form too, e.g. (/ *errs 100 tot). That's the recommended disambiguation shape, so flagging it contradicts the hint's own advice and spams every rate-calc program on every run. Streaming-tail rerun4 surfaced this as a false-positive papercut.

Repro

Before:

$ ilo 'rate errs:n tot:n>n;(/ *errs 100 tot)' rate 3 12
25
{"hints":["hint: `/*a b c` parses as `(a*b)/c` (inner prefix op binds first). For the other order, swap the ops (`*/a b c`) or bind: `r=/a b;*r c`"]}

After:

$ ilo 'rate errs:n tot:n>n;(/ *errs 100 tot)' rate 3 12
25

Bare unparenthesised form still fires as before.

What's in the diff

• diag: suppress prefix-precedence hint inside explicit parens — in detect_prefix_precedence_trap (src/main.rs), skip when the immediate predecessor of op1 is LParen. Token-local, one extra guard. Covers nested parens (((*/ a b c))) because the immediate predecessor is still LParen. Adds four unit + integration tests: paren-grouped pair (all four shapes), nested parens, bare form still fires, unit-level detector pin.
• example: parenthesised prefix pair runs silent — examples/prefix-pair-in-parens.ilo exercises the rate-calc shape under the engine harness via -- run: / -- out: assertions.

Test plan

• cargo build --release --features cranelift
• cargo test --release --features cranelift — full suite, 0 failures
• cargo clippy --release --features cranelift --all-targets -- -D warnings — clean
• cargo fmt --check — clean
• Manual repro: (/ *errs 100 tot) silent, bare */a b c still emits the hint
• All four pair shapes (*/, /*, +-, -+) suppressed inside parens
• Nested parens ((*/ a b c)) suppressed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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