Warn (don't abort) on a compile-time-constant WHEN

[g5t]

A constant WHEN condition used to raise RuntimeError from Instance.WHEN, aborting the whole parse/translate. Per #318, "turning off" a component while keeping its position/orientation/DISPLAY is a legitimate thing to want, so this is now a logger.warning that keeps the instance and points at the alternatives (remove/comment the component, or gate it on an instrument parameter).

Three related fixes:

• Expr.is_constant (and .value) recognise a folded boolean literal (0 == 1 -> False, 2 > 1 -> True); SymPy's BooleanAtom reports is_number == False, so these previously slipped past every constant check even though Discrepancy between Expr parsing of 1==0 vs 0==1 #317 folds them.
• Instance.WHEN / Instance.SPLIT coerce a non-str/non-Expr argument (e.g. an int) via Expr.best instead of raising.
• Assembler.component routes its when=/split= kwargs through Instance.WHEN/SPLIT rather than the raw struct constructor, so a string is parsed to an Expr (was stored verbatim, which broke parameter_used and the {when:p} translation format).

Fixes #318