feat: add default value support for primitives
Map TypeSpec default values to Lexicon default field:
- string: `name?: string = "value"` → `"default": "value"`
- integer: `age: int32 = 0` → `"default": 0`
- boolean: `active: boolean = true` → `"default": true"`
Implementation:
- Read `prop.default.value` from ModelProperty
- Emit default field on string, integer, and boolean primitives
- Type-safe: only emit if value type matches primitive type
Tests: All 23 existing tests remain green (no fixtures changed)
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Co-Authored-By: Claude <noreply@anthropic.com>