Type-check expectation arguments when precept is disabled

[carlsverre]

Problem

When the enabled feature is off, every macro in macros_stubs.rs expands to {}, discarding its condition and details expressions entirely. That has two consequences:

1. Arguments aren't type-checked in the disabled build. A typo or type error inside an expect_*! / observe! block only surfaces when the crate is compiled with enabled. This defeats the "same source compiles identically in every configuration" guarantee that makes observe! blocks safe to leave inline in the system under test.

2. Temporaries that exist only to feed an expectation become unused. In a downstream crate built with -D warnings, a perfectly ordinary pattern warns (and fails CI):

   observe!(|| {
       let sum: u64 = inputs.iter().sum();          // only used by the assertion
       expect_always!(sum == expected, "conserved", { "sum": sum });
   });

   With enabled off, expect_always! vanishes and sum is now unused_variables / unused_assignments.

Fix

Rewrite each disabled stub to reference its arguments inside a dead if false { … } block:

macro_rules! define_and_emit_entry {
    ($expectation:expr, $property:expr, $condition:expr $(, $($details:tt)+)?) => {
        if false {
            let _ = $expectation;
            let _ = &$property;
            let _ = $condition;
            $( let _ = $crate::deps::serde_json::json!($($details)+); )?
        }
    };
}

if false { … } is exactly the right tool here:

• Zero runtime overhead — the branch is provably unreachable and eliminated before codegen, so the arguments are never evaluated at runtime. The crate's "0 runtime overhead when disabled" promise is preserved.
• Still fully type- and borrow-checked — dead code is still checked, so a type error in an expectation now fails to compile in every configuration.
• Marks captures as used — let _ = $condition / json!($details) reference every temporary, silencing both unused_variables and the liveness assigned…never read lints.

Argument evaluation mirrors the enabled path in macros.rs (by value, same moves/borrows), so anything that compiles with enabled also compiles without it.

Only macros_stubs.rs changes; the enabled path (macros.rs) is untouched.

Verification

• cargo test passes with and without enabled.
• cargo clippy --all-targets -- -D warnings clean with and without enabled (modulo a pre-existing fault.rs assertions_on_constants lint unrelated to this change).
• A type error inside a disabled expect_always! still fails to compile (E0277).
• A downstream crate whose observe! block has an assertion-only temporary no longer warns under -D warnings, in both feature configs.

🤖 Generated with Claude Code