Add per-type accessors (bezier_params, power, speed, frequency,
pulse_width, laser_uid, layer_uid, workpiece_uid, section_params,
extra_axes, dwell_duration) and a linearize() method to Ops.
Migrate gcode, vertex, cairo, and ruida encoders from Command
object dispatch to CommandType/CommandCategory-based dispatch.

Port all transformers, producers, simulators, encoders, and ~700+
tests from direct Command/ops.commands usage to the index-based Ops
API. Add lazy state computation in clipping._get_state() to avoid
expensive preload_state() call. Update tracking docs to reflect
remaining work.

state_at(), preloaded_state() now return MachineState NamedTuple.
inspect() returns CommandInfo NamedTuple. This makes the read API
PyO3-friendly as required for the upcoming Rust port.

Inline group_by_state_continuity and flip_segment into Ops methods.
Delete group.py and flip.py. Refactor estimate_time to use Ops index
API instead of raw Command list. Add 42 tests covering 19 previously
untested public Ops methods.