## Add default parameter values for function definitions

### Summary

This PR adds support for default parameter values in function definitions, reducing boilerplate and improving API ergonomics:

```faust
// Define with defaults
lowpass(freq, q=0.707) = fi.resonlp(freq, q, 1);

// Call - omit defaulted parameters
process = lowpass(1000);        // q = 0.707
process = lowpass(1000, 1.5);   // q = 1.5
```

### Motivation

Currently, Faust functions require all parameters at every call site, even when sensible defaults exist:

```faust
// Without defaults - must specify q every time
lowpass(freq, q) = fi.resonlp(freq, q, 1);
process = lowpass(1000, 0.707), lowpass(2000, 0.707), lowpass(500, 0.707);

// With defaults - cleaner code
lowpass(freq, q=0.707) = fi.resonlp(freq, q, 1);
process = lowpass(1000), lowpass(2000), lowpass(500);
```

This could be especially valuable for library functions where most users want standard values but experts need tunability.

### Syntax

```faust
// Single default
gain(x, amount=1.0) = x * amount;

// Multiple defaults (must be rightmost parameters)
mix(a, b, ratio=0.5) = a * (1-ratio) + b * ratio;
env(attack=0.01, decay=0.1, sustain=0.7, release=0.3) = ...;

// Defaults can be expressions
SR = 48000;
nyquist(sr=SR/2) = sr;

// Defaults can reference earlier parameters
scale(x, factor, offset=factor*0.1) = x * factor + offset;
```

**Constraints:**
- Parameters with defaults must come after parameters without defaults
- Default expressions are evaluated at definition time

### Implementation

- **Parser**: Extended function parameter syntax to accept `= expression`
- **Boxes**: Default values stored by position (nil for required params)
- **Eval**: Modified function application to fill missing arguments from defaults

### Testing

Pass tests:
- `default-params-basic.dsp` - single default parameter
- `default-params-multiple.dsp` - multiple defaults in one function
- `default-params-expression.dsp` - defaults using expressions/constants

### Note on diff size

The large diff in generated files (`faustparser.cpp`, etc.) is from flex/bison regeneration. Actual source changes:

| File | Lines changed |
|------|---------------|
| `faustparser.y` | +97 |
| `global.hh` | +6 |
| `sourcereader.cpp` | +16/-9 |

### Backward Compatibility

Fully backward compatible - existing function definitions without defaults work unchanged.

### Future Work

Combines naturally with keyword arguments (separate PR):

```faust
filter(freq, q=0.707, gain=1.0) = ...;

// All these work:
process = filter(1000);
process = filter(1000, 1.5);
process = filter(1000, gain: 2.0);        // skip q, override gain
process = filter(freq: 1000, gain: 2.0);  // named + default
```