feat: Add SQLite extension loading support

[dsisnero]

Summary

Add enable_load_extension and load_extension methods to SQLite3::Connection for dynamically loading SQLite extensions at runtime.

Closes #106

Changes

• Add lib bindings for sqlite3_enable_load_extension and sqlite3_load_extension
• Add Connection#enable_load_extension(enabled : Bool) method
• Add Connection#load_extension(path, entry_point) method with full documentation
• Add TDD tests for extension loading (6 tests, all pass)

Use Cases

SQLite extensions provide valuable functionality:

• sqlite-vec: Vector similarity search for AI applications
• sqlite-json: Enhanced JSON support
• sqlite-fts5: Full-text search
• Custom domain-specific extensions

Without extension loading, Crystal applications cannot leverage these powerful SQLite extensions.

Testing

# All 336 existing tests pass
crystal spec

# New extension loading tests pass
crystal spec spec/extension_loading_spec.cr
# 6 examples, 0 failures, 0 errors, 0 pending

Example Usage

db = DB.open("sqlite3::memory:")
db.using_connection do |conn|
  sqlite_conn = conn.as(SQLite3::Connection)
  sqlite_conn.enable_load_extension(true)
  sqlite_conn.load_extension("/path/to/extension.dylib")
end

[ysbaddaden]

Issue: unrelated change.

[ysbaddaden]

Issue: Wrong spec file. Not needed, either.

[ysbaddaden]

Issue The whole specs are wrong.

The really interesting spec will only run on macOS with sqlite3 installed through Homebrew, and otherwise silently skips (false success). At the very least a skipped spec must be explicit.

Maybe we could compile a dummy extension, or take an ENV parameter to an extension library, so we can at least configure CI.

There are no specs for testing that we can't load an extension when disabled.

There are also no specs that try to execute a query that uses the extension and asserts that it succeeds (enabled) or fails (not loaded).

[ysbaddaden]

Issue: memory leak, err_msg has been allocated and must be freed.

Suggested change

	err_msg = Pointer(UInt8).null
	result = LibSQLite3.load_extension(@db, path, entry_point, pointerof(err_msg))
	if result != 0
	msg = err_msg.null? ? "Unknown error" : String.new(err_msg)
	raise Exception.new("Failed to load extension: #{msg}")
	end
	ret = LibSQLite3.load_extension(@db, path, entry_point, out err_msg)
	return unless ret == 0
	msg = err_msg ? String.new(err_msg).tap { LibSQLite3.free(err_msg) } : "Unknown error"
	raise Exception.new("Failed to load extension: #{msg}")

[ysbaddaden]

Issue: this is invalid.