opal-zig v2.0.0-1

Changed

• Binds opal 2.0.0. Rendered output is bit-identical to 1.0.2, only the API shape
  changed.
• An Opal instance is now a plain value: upstream replaced every internal pointer
  with an index, so copying, assigning, or relocating an instance is legal and a copy
  is a complete save state. The no-copy rule is gone.
• Opal.init returns the instance by value. create and destroy remain as heap
  helpers for the roughly 70 KiB struct.
• Methods follow the upstream renames: port is now writeReg, portBuffered is
  writeRegBuffered, read is readStatus. The raw externs are opalInit,
  opalWriteReg, and so on.
• Constants follow the upstream renames: num_channels is now channel_count and
  num_operators is operator_count.
• Struct mirrors follow the new pointer-free layout: op_slot, chan_index,
  mod_source, pair_index, and out_source are index fields, boolean fields are
  bool, and Opal.zero_mod is gone.

Added

• EnvelopeStage and ChannelType, non-exhaustive enum(c_int) types for the
  envelope_stage and chan_type fields.
• Index sentinels op_none, mod_own_fb, and ch_none.
• A save-state test: a copied chip renders the same audio as the original.

Removed

• The Bool alias for opal_bool, which upstream removed in favor of C11 bool.
• The optional cross-pointer fields (master, chan, mod, channel_pair,
  out_ptr), replaced by the index fields above.