fix(cli): synth verify exits non-zero when built without verify feature

[avrabe]

Summary

Closes #124. synth verify is advertised in synth --help and the usage examples, but on a binary built without the verify feature it printed a "rebuild" hint and returned Ok(()) — exit code 0. A script or CI step gating on synth verify would silently believe the binary was validated when nothing ran.

synth verify is the Z3 translation-validation oracle — the "did the ARM lowering preserve WASM semantics" check, load-bearing on the ASIL-D path. A success-shaped no-op defeats it.

Fix

The #[cfg(not(feature = "verify"))] branch now anyhow::bail!s with a clear message and a non-zero exit:

Error: this `synth` binary was built without the `verify` feature — Z3 translation
validation is unavailable.
  Rebuild with verification support:
    cargo build --features verify
  (or `cargo install --path crates/synth-cli --features verify`)

Also marks the Verify subcommand's help text as requiring --features verify.

What's deliberately NOT changed

verify stays a non-default feature. Making it default would pull z3-sys into every cargo install — z3-sys needs network at build time, which is exactly why it's opt-in. The fix is to fail honestly, not to force the dependency on everyone.

Test plan

• cargo install --path crates/synth-cli (no verify) → synth verify a.wasm b.elf exits non-zero with the clear message.
• cargo install --path crates/synth-cli --features verify → synth verify runs Z3 validation as before.
• synth --help shows the --features verify requirement on the Verify subcommand.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/synth-cli/src/main.rs	0.00%	1 Missing ⚠️

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