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CSE machine: allocate local variables as unassigned at CALL time (#235)

* CSE machine: allocate local variables as unassigned at CALL time (#223)

Locals were previously bound lazily whenever their assignment happened
to execute, so a read that races ahead of its binding (temporal dead
zone) fell through the empty frame to an unrelated same-named binding
further up the environment chain instead of raising an error.

createEnvironment now preallocates every local of a function frame
(assignment/def/for targets, minus params) as an UNASSIGNED sentinel
at CALL time. pyGetVariable/pyGetNonlocalVariable now check for that
sentinel and raise UnboundLocalError (own frame) or
FreeVariableUnboundError (enclosing frame) accordingly, matching
CPython's behavior.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Use explicit else in pyGetVariable's unassigned-value branch

handleRuntimeError always throws (return type never), so this was not
a functional bug, but the missing else made the control flow read as
if both errors could fire. Addresses gemini-code-assist review comment
on PR #235.

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Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

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