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CSE machine: if/elif and conditional expressions require a strict bool condition (#467)

* CSE machine: if/elif and conditional expressions require a strict bool condition

Every spec doc, under "Conditional statements and conditional
expressions", says: "Following if and elif, Python §x only allows
boolean expressions." A companion PR (#439) already tightened this for
the py2js engine and the substitution-model stepper, mirroring the
treatment and/or/not and (separately) while already got. The CSE
machine's BRANCH instruction (src/engines/cse/interpreter.ts) applied
isFalsy's general any-type truthiness unconditionally instead, so e.g.
`if 1: ...` or `1 if "x" else 2` silently took the truthy branch
rather than raising, same as native Python -- unlike this dialect's
own spec.

BRANCH is used by both if/elif and the conditional expression, so one
check-site covers both (matching py2js's shared condBool / the
stepper's shared contractConditional): if the popped condition isn't
already type "bool", raise before falling through to the existing
isFalsy-based branch selection. Distinguishes if vs. ternary only for
the error's wording ("if condition" vs. "conditional expression
condition"), reading instr.srcNode -- already available on every
instruction via BaseInstr, no new field needed.

Added a new ConditionNotBoolError (errors.ts), modeled closely on the
existing ListIndexTypeError/UnsupportedOperandTypeError templates:
neither of those fit directly, since UnsupportedOperandTypeError's
node type is Binary/BoolOp/Unary-only and dereferences node.operator,
which If/Ternary don't have. Uses CSE's own established "friendly type
name, no quotes" phrasing (matching UnsupportedOperandTypeError's
"bad operand type for unary not: boolean" style) rather than
byte-matching py2js/stepper's "predicate type must be 'bool', not
'int'" wording -- per the issue's own allowance for "equivalent
CSE-style wording."

Updates py2js-conditional-bool.test.ts, whose header comm... (continued)

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