Implement module system and program flattening

[LesterEvSe]

Overview

Implements the module system proposed in #318, enabling multi-file programs to be compiled into a single entry point for browser execution via Simplex.

Changes

• Driver: Each imported file is now wrapped in a mod unit_N { ... } block during flattening, preserving namespace boundaries and visibility rules
• Analyzer: added ModuleScope and scope-aware resolution to handle nested modules, visibility checks, and use rewriting across module boundaries

Supported Syntax

pub mod foo { ... }          // visibility
mod outer { mod inner { } }  // nesting
use crate::foo::bar;         // explicit import

Related PRs (in order of the merge): #338, #339, #343, #345

[LesterEvSe]

Next Steps After Merging:
We expect the modules feature to be marked as unstable in PR #329.
After merging this and the Unstable Features PR, we plan to bump the SimplicityHL version to 0.6.0-rc.1.

[KyrylR]

ACK 4359a84; based on the ACKs in: #338, #339, #343, #345