feat: stabilize wavec for Vex tooling

[LunaStev]

Summary

• Adds machine-readable wavec print ... --format=json output for Vex and other build tools.
• Adds JSON CLI/front-end diagnostics through --error-format=json.
• Extends Wave's special #[target(...)] condition syntax to support arch, os, env, and abi.
• Prevents x86_64-only macOS raw syscall std modules from being selected on non-x86_64 macOS targets.
• Documents Wave's release-count versioning policy and bimonthly release cadence.

Why

Vex needs a stable compiler interface instead of parsing human text. The compiler should expose supported targets, emit kinds, input kinds, target metadata, and dry-run plans in a predictable format.

Validation

• cargo fmt --all --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo test --locked --all-targets
• python3 -m py_compile x.py tools/run_tests.py
• cargo build --locked --release
• python3 tools/run_tests.py (96 pass, 12 skip, 0 fail; test56.wave skipped locally because TCP sockets are blocked by this environment)