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Stepper: self-referential declarations bind cyclically (#497) (#498)

* Stepper: self-referential declarations bind cyclically (#497)

`b = pair(1, lambda: b)` substituted `b` only into the rest of the
program, leaving the `b` inside its own value free, so calling the
lambda raised NameError. Following the Source stepper spec's
substitution rule, a name free in its own (non-function) value now
refers cyclically to that value (`tieKnot`).

Makes the AST walkers that can reach such a value cycle-safe: clone,
substitute, freeNames, the capture-avoiding rename, allIdentifierNames,
markBuiltins and unparse (a back-reference renders as its name).

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

* Stepper: tieKnot visits each object once

A value that already contains a cycle (tied by an earlier declaration,
e.g. `a = pair(1, lambda: a); b = pair(a, lambda: b)`) sent tieKnot's
patch walk around that cycle forever. Adds a regression test, plus one
pinning that an eager self-reference (`x = pair(1, x)`) is still a
NameError.

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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