## Add -il option to inline all libraries

**Discussion**: https://github.com/grame-cncm/faust/discussions/832

### Summary

This PR adds a new `-il` (`--inline-libraries`) option that produces a self-contained DSP file with all library code inlined:

```bash
faust -il myfile.dsp -o myfile_standalone.dsp
```

The output preserves readable Faust source code, unlike `-e` which exports an unreadable expanded box tree. **Tree-shaking** automatically removes unused definitions to keep output minimal.

### Motivation

From issue #832: Users need self-contained DSP files for sharing, archiving, or using in environments without library access. The existing `-e` option exports expanded DSP but produces unreadable output.

### Example

**Input (`mytest.dsp`):**
```faust
import("mymath.lib");
fx = library("myeffects.lib");
process = _ : half : fx.gain;
```

**mymath.lib:**
```faust
half(x) = x * 0.5;
double(x) = x * 2;  // unused by this program
```

**Output with `-il`:** (note: `double` is removed by tree-shaking)
```faust
// Self-contained DSP file generated with -il option
// Inlined libraries: 2
// Tree-shaking: enabled (5 definitions used)

// ===== Imported: mymath.lib =====
half(x) = x * 0.5;
// ===== End of: mymath.lib =====

// Library: mymath.lib
__lib_mymath = environment {
half(x) = x * 0.5;
};

// Library: myeffects.lib
__lib_myeffects = environment {
gain = *(0.8);
};

// ===== Main DSP =====
fx = __lib_myeffects;
process = _ : half : fx.gain;
```

### How It Works

1. **Evaluation phase**: Runs normal Faust evaluation, tracking which definitions are used
2. **Collection phase**: Recursively discovers all `import()` and `library()` dependencies
3. **Output phase**:
   - `import()` content inlined directly at top level (into global scope)
   - `library()` content wrapped in `environment { }` as top-level definitions
   - **Tree-shaking**: Only used definitions are included in output
   - Files used as both `import()` and `library()` are handled correctly
   - Circular dependencies detected and reported
   - Duplicate variable definitions skipped

### Tree-Shaking

The `-il` option automatically removes unused definitions by:
1. Evaluating the program first to determine which symbols are actually used
2. Tracking all symbol lookups during evaluation in `gUsedDefinitions`
3. Filtering the output to only include definitions whose names appear in the tracked set

This significantly reduces output size when importing large libraries but only using a few functions.

### Implementation

As suggested in the original discussion, this reuses infrastructure from the `-i` option:
- Uses `gGlobal->gAlreadyIncluded` set for tracking visited files
- Uses `fopenSearch()` for resolving library paths
- Similar recursive inject pattern

**Why new code was needed on top of `-i`:**

The `-i` option handles C++ `#include` in architecture files with simple text substitution. The `-il` option needs Faust-specific semantics:

| `-i` (architecture) | `-il` (libraries) |
|---------------------|-------------------|
| Detects `#include <faust/...>` | Must parse `import("...")` and `library("...")` |
| Just copies file content | `import()` → inline directly |
| | `library()` → wrap in `environment { }` |
| Single use pattern | Files can be used as both import AND library |
| | Must track variable definitions to avoid duplicates |
| | Tree-shaking removes unused definitions |

| File | Changes |
|------|---------|
| `global.hh` | +2 lines (flags) |
| `global.cpp` | +8 lines (option parsing, init, help) |
| `libcode.cpp` | +15 lines (integration) |
| `sourcereader.hh` | +3 lines (declaration) |
| `sourcereader.cpp` | +280 lines (inlining + tree-shaking logic) |
| `eval.cpp` | +8 lines (symbol tracking) |

### Testing

```bash
# Simple test
faust -il test.dsp -o test_inlined.dsp
faust -e test_inlined.dsp  # Verify it compiles

# Tree-shaking in action
faust -il uses_one_function.dsp -o small_output.dsp
```

### Option Naming

Follows the pattern of `-it` (`--inline-table`):
- `-il` = inline libraries
- `-it` = inline table

### Backward Compatibility

Fully backward compatible - new option, no changes to existing behavior.

### Branch

- **Worktree**: `../faust-inline-libs`
- **Branch**: `feature/inline-libraries`
- **Base**: `master-dev`