Add multi-file compilation, imports, visibility modifiers, and test runner

[benpayne]

Summary

This PR adds support for multi-file module compilation with import statements, visibility modifiers (pub), and a test discovery/runner system. The compiler can now parse multiple input files as separate modules with proper import resolution, and the bcc driver includes a test subcommand for discovering and validating .b files.

Key Changes

Module System & Imports

• Added ImportStatement class to represent import module_name; declarations
• Implemented import statement parsing with support for dotted paths (import std.io;)
• Each input file is now parsed as a separate module with its own scope parented to the global scope
• Module-level scopes isolate top-level symbols while sharing built-in types

Visibility Modifiers

• Added pub keyword support for marking functions, structs, enums, and protocols as public
• Functions and structs now track mIsPublic flag during parsing
• Default visibility is private; pub is required for cross-module access
• Updated FunctionDefinition::Parse and StructDefinition::Parse to accept isPublic parameter

Compiler Driver (qcc)

• Extended to accept multiple input files: qcc file1.b file2.b ...
• Added command-line options: -S/--emit-ir, -c/--emit-obj, -o/--output, --parse-only, -h/--help
• Each module generates its own .ll file (or uses -o for the first file)
• Code generation now processes all modules in sequence

Test Infrastructure (bcc)

• Added bcc test subcommand for discovering and running test files
• Discovery strategy: searches tests/ directory if present, otherwise current directory for *.b files
• Runs each file through qcc for parse-only validation
• Reports pass/fail summary with exit code indicating test results
• Added --verbose flag for detailed output

Language Features

• Added ? (try/propagate) operator for error handling (lexer support)
• Added TryExpression AST node for postfix ? on expressions
• Documented flat namespace constraint: no nested modules, no qualified-name expressions in source
• Documented mandatory explicit type signatures on public functions (already enforced by grammar)

Scope Management

• Impl blocks now use local scopes to avoid polluting global scope
• Protocol method signatures use local scopes to prevent conflicts with impl methods
• Scope::addSymbol now returns bool to detect duplicate symbol definitions

Documentation & Testing

• Updated language_design.md with import syntax, visibility rules, and flat namespace explanation
• Added comprehensive test files for new features:
  • import_basic.b, import_dotted.b (pass)
  • import_missing_name.b, import_missing_semi.b (fail)
  • visibility_basic.b, pub_function.b, pub_struct.b, pub_enum.b (pass)
  • protocol_conformance.b, protocol_missing_method.b (pass/fail)
  • try_operator.b, result_option.b, match_enum_variants.b (pass)
  • duplicate_func.b (fail) — now tests actual function name conflicts
• Moved extern_func_call.b from xfail to pass (now properly supported)
• Updated implementation_plan.md with current task status (62 pass tests, 21 fail tests)

CI/CD

• Added GitHub Actions workflow (.github/workflows/ci.yml) with parse-only and with-llvm matrix builds
• Updated install_deps.sh to support cross-platform dependency installation with optional LLVM

Implementation Details

• Import statements are stored in Module::mImports vector but not yet enforced during symbol resolution (deferred to Task 63)
• The flat namespace is enforced by grammar: the parser never produces qualified-name expression nodes
• Public function type signatures are automatically valid (no inference syntax exists in BLang)
• Test discovery uses find command

https://claude.ai/code/session_01Qum6HxALUGHyurMWLrH9G4