feat: add an xcode-xcstrings-v2 bucket that supports CLDR pluralization rules (#1230)

* feat: add xcode-xcstrings-v2 bucket with ICU MessageFormat support

Add opt-in xcode-xcstrings-v2 bucket type that converts CLDR plural forms
to ICU MessageFormat for better translation quality and variable handling.

## Key Features

- **ICU MessageFormat conversion**: Converts xcstrings plural forms to ICU
  format with metadata preservation
- **Backward compatible**: Original xcode-xcstrings (v1) completely unchanged
- **Opt-in migration**: Users explicitly choose v2 in their config
- **Format preservation**: Maintains Objective-C format specifiers (%d, %.1f, %lld, %@)
- **Variable handling**: Supports multiple variables with proper metadata
- **CLDR support**: Handles all plural categories (zero, one, two, few, many, other)

## Implementation

### New Files
- xcode-xcstrings-icu.ts - Core ICU conversion utilities (567 lines)
- xcode-xcstrings-v2-loader.ts - V2 bucket loader (79 lines)
- xcode-xcstrings-icu.spec.ts - 23 ICU conversion tests
- xcode-xcstrings-lock-compatibility.spec.ts - 5 lock format tests
- icu-safety.spec.ts - 22 safety tests for other bucket types

### Modified Files
- loaders/index.ts - Register xcode-xcstrings-v2 bucket type
- loaders/flat.ts - Add shouldPreserveObject option for ICU objects
- loaders/variable/index.ts - ICU-aware metadata stripping
- spec/formats.ts - Register xcode-xcstrings-v2 format

### Demo
- demo/xcode-xcstrings-v2/ - Example files with ICU pluralization

## Lock File Format

V1 (flattened):
```
item_count/one: <checksum>
item_count/other: <checksum>
```

V2 (ICU object):
```
item_count: <checksum of ICU object>
```

## Test Coverage

- 72 total tests (50 new + 22 existing)
- All 430 tests passing
- Safety verified against 25+ bucket types

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

* chore: fix formatting

* chore: cleaning

* chore: add changeset

* chore: fix formatting

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: vrcprl

.changeset/forty-readers-shop.md

@@ -0,0 +1,7 @@
+---
+"@lingo.dev/_spec": patch
+"lingo.dev": patch
+"@lingo.dev/_sdk": patch
+---
+
+add an xcode-xcstrings-v2 bucket type that supports cldr pluralization rules