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WASM: if/elif and conditional expressions require a strict bool condition (#466)

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. WASM's
visitIfStmt/visitTernaryExpr routed the condition through BOOLISE_FX
(the general any-type-truthy helper) instead of CHECK_BOOL_FX (the
strict-bool one and/or's left operand and not already use) -- so
`print(1 if 5 else 2)` silently printed 1 instead of raising a
TypeError.

Fixed using the exact CHECK_BOOL_FX-then-BOOLISE_FX composition
visitBoolOpExpr already uses for and/or's left operand: validate via
CHECK_BOOL_FX first (faults with the existing EXPECTED_BOOL_OPERAND
error if not a bool), then let BOOLISE_FX extract the now-known-bool
truthy value for the wasm `if`. Reuses WASM's existing generic error
message rather than inventing new parametrized wording, matching the
issue's own allowance to follow "WASM's existing error-reporting
conventions."

visitWhileStmt has the identical gap but is deliberately left alone --
out of scope for #437, matching the companion py2js/stepper fix's own
scoping (tracked separately).

Updates the two "if condition uses truthiness" tests and the "ternary
uses truthiness" test in wasm-control-flow.spec.ts (previously pinning
the any-type behavior as expected) to assert EXPECTED_BOOL_OPERAND
instead, matching the established assertion style already used for
and/or/not's identical check in wasm-operators.spec.ts. Also fixes the
CHECK_BOOL_FX doc comment in runtime/operators.ts, which listed "if"
alongside "while" as a BOOLISE_FX-truthiness context -- true before
this change, not after.

Verified: full WASM suite (9 suites, 454 tests) passes; the issue's
own repro (`print(1 if 5 else 2)`) now raises; `while` with a non-bool
condition still runs via truthiness, confirming it's untouche... (continued)

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