Copy embedded engine options before supplying defaults

[OskarEichler]

Summary

Copy an embedded engine's option hash before adding the default encoding. Rendering should not mutate the caller's options; frozen configuration should work too.

Reproduction

require 'slim'
options = { auto_ids: false }.freeze
Slim::Template.new(markdown: options) { "markdown:\n  # Header" }.render

With Kramdown available, this currently raises FrozenError while assigning default_encoding. With mutable options it silently adds a key to the caller's hash. The patch renders the same output and leaves both frozen and mutable input unchanged. Existing explicit encoding/default behavior is retained.

Verification

• Unmodified upstream core/plugin suites: 415 runs / 650 assertions, zero failures/errors/skips on Ruby 3.2.11 and 4.0.6.
• Existing Rails integration: 15 runs / 36 assertions passing on each, Rails 8.1.3.1 / Temple 0.10.7 / Tilt 2.9.0.
• 720 focused checks per Ruby across frozen/mutable hashes, explicit/missing/nil/false encodings and Markdown options.
• Ruby syntax passes; comparative Lint introduces no new offenses (existing baseline offenses retained). No test files changed.

Compatibility / limitations

No API removal, dependency upgrade or intended breaking change. This is a shallow copy because only the top-level default is assigned. Kramdown/Asciidoctor and the existing suites were exercised; not every optional Tilt engine or Ruby implementation.