Complete the Wave module import system

[LunaStev]

Summary

• replace flat source inclusion with a module graph supporting package roots, package submodules, local aliases, selective imports, and explicit public re-exports
• enforce declaration visibility across module boundaries, reject pub fun main(), and keep entry points private
• diagnose private access, namespace conflicts, import cycles, and package-root escapes while mangling imported definitions safely
• support inferred var initializers and empty-struct construction used by the documented import examples
• migrate the standard library, examples, and regression fixtures to the explicit module contract

Package contract

• import("add") resolves the Vex package root src/lib.wave
• import("add::math") resolves src/math.wave
• import("./helpers" as helpers) creates a local alias
• import("add")::{sum, Point} imports only public symbols

Validation

• cargo fmt --all --check
• cargo check --locked --jobs 2
• cargo test --locked --jobs 2 (39 root tests passed)
• cargo test -p parser --locked --jobs 2 (6 parser tests passed)
• RUSTDOCFLAGS="-D warnings" cargo doc --locked --no-deps --jobs 2
• cargo clippy --locked --all-targets --jobs 2 -- -D warnings
• ./tools/check_std_policy.sh
• full language suite: 100 passed, 8 platform skips, 0 failed
• path and local Git Vex dependency end-to-end checks

Companion change

• Enforce canonical Wave library dependencies Vex#17 defines and tests the matching package-manager contract.