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e-stepper: environment stepper engine for Python §3/§4 (#513)

* e-stepper: environment stepper engine for Python §3/§4 (#508)

The program is rewritten step by step as in the substitution stepper, but
names are looked up in environments, a call to a user function steps to its
body annotated with a new frame (EnvBlock), assignments change frames, and
lists and function objects are heap objects the program refers to (Ref).

The store is the CSE machine's: frames are CSE Environments (created with
createEnvironment), function objects are Closures, values are CSE Values,
and operators, lookups, list access/assignment and the stdlib are the CSE
machine's own functions, so results and errors match the CSE machine.
Library prelude functions are applied atomically on the CSE machine.

Adds PyEStepperEvaluator3/4, the PyEStepperRunnerPlugin (on
@sourceacademy/runner-e-stepper), and yarn repl --engine estepper [--trace].

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* e-stepper: input() asks the host, or fails clearly without one

The e-stepper's stdin was a closed stream, so input() silently read "" and
the program continued. Connect it like the CSE evaluator's input stream:
the evaluator passes the host's requestInput through the runner plugin to
the engine. Without a host (CLI, tests), input() now stops evaluation with
an error instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

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