Remove let syntax and integrate v0.2.1 CI gates

[LunaStev]

Summary

• retire let and let mut declarations in favor of the Wave-native var syntax, including for initializers
• migrate maintained examples, standard-library modules, tests, and ABI fixtures to var
• add std policy, warning-free rustdoc, examples/std corpus, feature-matrix, and platform E2E report gates to CI
• require a dedicated RV64 ABI and hosted Linux/QEMU validation job before release packaging
• make macOS Intel CI blocking and preserve platform skip reasons as JSON artifacts

Testing

• cargo fmt --all --check
• cargo check --locked --jobs 2
• cargo clippy --locked --all-targets --jobs 2 -- -D warnings
• RUSTDOCFLAGS='-D warnings' cargo doc --locked --no-deps --jobs 2
• cargo test --locked --all-targets --jobs 2 (37 passed)
• cargo build --locked --release --jobs 2
• RISC-V-only and core64 LLVM feature builds
• RV64 QEMU contract tests (8 passed)
• maintained Wave corpus (92 passed)
• Wave E2E (100 passed, 8 platform/environment skips, 0 failed, 0 timed out)
• Python tooling tests, workflow YAML parsing, std policy check, and git diff --check