feat: Add config-based ATTACH DATABASE API

[enki]

Implements config-based ATTACH API with explicit attach()/detach() methods.

This supersedes #327 and implements the config-based approach suggested by maintainers in the previous PR discussion.

Changes

Core API (libsql-core)

• AttachConfig interface with alias and path fields
• attach?: AttachConfig[] option in createClient() config for static attachments
• client.attach(alias, path) and client.detach(alias) methods for dynamic attachments

Implementation (Sqlite3Client)

• Config-based static attachments applied at client creation
• Explicit attach/detach methods for runtime attachment
• Automatic persistence across transaction() and reconnect()
• Graceful handling of missing databases (logs warnings, doesn't crash)
• Read-only mode support via file:///path?mode=ro URIs

Remote Clients (HTTP/WebSocket)

• Stub implementations that throw ATTACH_NOT_SUPPORTED error
• ATTACH only works with local SQLite files

Tests

• 13 comprehensive tests covering all use cases
• Config-based attachments
• Explicit attach/detach methods
• Persistence across transactions and reconnects
• Read-only mode
• Cross-database JOINs
• Error handling (duplicate aliases, missing files)
• Backward compatibility

Design Principles

Zero overhead: No SQL parsing or regex matching. Users explicitly control when and what to attach.

Persistence: Attachments survive transaction() (which nulls the connection) and reconnect() by re-applying from stored config.

Unified storage: Single internal array stores both config and explicit attachments, ensuring consistent behavior.

Addresses feedback from #327

• ✅ Zero overhead (no SQL parsing)
• ✅ User-controlled explicit API
• ✅ Supports both static (config) and dynamic (explicit) attachments
• ✅ Read-only mode for preventing write locks
• ✅ Graceful degradation (missing DBs log warnings)

Example Usage

Static attachments:

const client = createClient({
  url: 'file:///path/to/main.db',
  attach: [
    { alias: 'archive', path: 'file:///path/to/archive.db?mode=ro' },
    { alias: 'analytics', path: 'file:///path/to/analytics.db' }
  ]
});

// Attachments persist across transactions
const tx = await client.transaction();
await tx.execute('SELECT * FROM archive.old_messages');

Dynamic attachments:

const client = createClient({ url: 'file:///path/to/main.db' });

// Attach at runtime
await client.attach('logs', 'file:///path/to/logs.db?mode=ro');

// Use in queries
await client.execute('SELECT * FROM logs.entries WHERE date > ?', [yesterday]);

// Detach when done
await client.detach('logs');

Closes #327

[enki]

@penberg this supersedes #327 based on your suggestions - can you review?