refactor(compiler): build semantic module graph

[fffonion]

Summary

• Builds deterministic module and symbol graphs.
• Preserves nested import correctness, diagnostics, source maps, CLI behavior, wasm, and no-std compatibility.
• Folds the semantic-module follow-ups into one implementation commit.

Stack

• Base: plan(agent): consolidate core runtime roadmap #14
• Next: refactor(vm): unify host runtime lifecycle #16

Verification

• Compiler suite passed
• no-std focused suite passed
• One implementation commit