feat: Std::String, Std::Map, Result/Option, ? operator, sizeof fix, docs

Add standard library modules:
- Std::String: strlen, strcmp, concat, copy, starts_with wrappers
- Std::Map: linear-probing hash map with String keys, int values

Add error handling:
- Result and Option algebraic enum examples
- ? try operator for automatic error propagation in HIR lowering

Compiler bugfixes:
- Fix sizeof() for user-defined structs (new hSizeOf HIR node)
- Fix HIR type lowering for char8, bool8, int8, uint8, etc.
- Fix let statement lowering for pointer types (*char8 → int* bug)
- Fix PrintInt ABI mismatch (int64_t → int in io.c)
- Fix Makefile test bug (_test_tmp_pkg already exists)

Documentation:
- Rewrite README.md with features, examples, project structure
- Add docs/LanguageRef.md — complete language reference
- Add docs/Stdlib.md — standard library documentation
- Add docs/BuildAndTest.md — build and test guide

New examples: strings, map, result_option, try_operator (13 total)

docs/BuildAndTest.md

@@ -0,0 +1,190 @@
+# Build and Test Guide
+
+This guide covers building the Bux bootstrap compiler, creating projects, and running tests.
+
+---
+
+## Prerequisites
+
+- **Nim** (1.6+) — for building the bootstrap compiler
+- **C compiler** (`gcc`, `clang`, or `cc`) — for the C backend
+- **Make**
+
+On Debian/Ubuntu:
+```bash
+sudo apt-get install nim gcc make
+```
+
+On macOS:
+```bash
+brew install nim gcc make
+```
+
+---
+
+## Building the Compiler
+
+```bash
+# Release build
+make build
+
+# Development build (no optimizations, faster compile)
+make dev
+```
+
+The output is a single binary: `buxc`.
+
+---
+
+## Creating a Project
+
+```bash
+# Create a new package in a new directory
+./buxc new myproject
+cd myproject
+```
+
+This generates:
+```
+myproject/
+├── bux.toml
+└── src/
+    └── Main.bux
+```
+
+### bux.toml
+
+```toml
+[Package]
+Name    = "myproject"
+Version = "0.1.0"
+Type    = "bin"
+
+[Build]
+Output = "Bin"
+```
+
+---
+
+## Building and Running
+
+```bash
+# Type-check without building
+./buxc check
+
+# Build
+./buxc build
+
+# Build and run
+./buxc run
+
+# Clean build artifacts
+./buxc clean
+```
+
+Build output goes to `build/` by default.
+
+---
+
+## Running Tests
+
+### Compiler Tests
+```bash
+make test
+```
+
+This runs:
+- Lexer unit tests
+- Parser unit tests
+- Semantic analysis unit tests
+- HIR lowering unit tests
+- Integration tests (`buxc new`, `buxc --version`)
+
+### Example Programs
+```bash
+make test-examples
+```
+
+Compiles and runs all programs in `examples/`.
+
+### Individual Example
+```bash
+mkdir -p examples_pkg/hello/src
+cp examples/hello.bux examples_pkg/hello/src/Main.bux
+# Create bux.toml manually or use `buxc new`
+cd examples_pkg/hello && ../../buxc run
+```
+
+---
+
+## Project Layout
+
+```
+bux/
+├── src/              # Bootstrap compiler source (Nim)
+│   ├── main.nim      # Entry point
+│   ├── cli.nim       # CLI commands
+│   ├── lexer.nim     # Tokenizer
+│   ├── parser.nim    # Parser
+│   ├── sema.nim      # Semantic analysis
+│   ├── hir.nim       # High-level IR
+│   ├── hir_lower.nim # AST → HIR lowering
+│   ├── c_backend.nim # HIR → C code generation
+│   └── ...
+├── stdlib/           # Standard library
+│   ├── Std/
+│   │   ├── Io.bux
+│   │   ├── Array.bux
+│   │   ├── String.bux
+│   │   └── Map.bux
+│   ├── runtime.c     # C runtime shim
+│   └── io.c          # C I/O functions
+├── examples/         # Example programs
+├── tests/            # Unit tests (Nim)
+├── docs/             # Documentation
+└── Makefile
+```
+
+---
+
+## Debugging
+
+### Verbose Output
+```bash
+./buxc build -v
+```
+
+### Inspecting Generated C
+```bash
+./buxc build
+cat build/main.c
+```
+
+### Common Errors
+
+| Error | Cause | Fix |
+|-------|-------|-----|
+| `stdlib directory not found` | `buxc` can't find `stdlib/` | Run from project root or set correct path |
+| `duplicate symbol 'bux_alloc'` | Multiple stdlib modules declare same extern | Only declare in one module |
+| `C compilation failed` | Generated C has errors | Check `build/main.c` for issues |
+
+---
+
+## Adding a New Example
+
+1. Create `examples/myexample.bux`
+2. Add `myexample` to `EXAMPLES` in `Makefile`
+3. Run `make test-examples`
+
+---
+
+## Development Workflow
+
+```bash
+# After making changes to the compiler:
+make build
+make test
+
+# If tests pass, run examples:
+make test-examples
+```