Propagate an undefined value for possibly-zero ///% divisors

[AaronWebster]

Part 2 of 3 of the integer division/modulus series.

Stacked on #273 (base branch: emboss/divmod-operators). Review/merge #273
first; the base retargets to master automatically once #273 merges. The diff
below is only this phase's changes.

Relaxes the strict division-by-zero rejection from part 1: a divisor that is
merely possibly zero is now allowed and propagates an undefined value
through the expression type system, instead of being a blanket compile error.
Only a provably always zero divisor (range exactly {0}, e.g. x // 0)
remains a hard error.

At runtime a // 0 (or % 0) yields an Unknown Maybe, which poisons the
enclosing expression -- a field's size, existence condition, or value -- so
Ok() becomes false with no undefined behavior, rather than the whole schema
failing to compile.

What this does

• ir_data.py: a separate can_be_undefined boolean flag on IntegerType
  and BooleanType (never an in-band sentinel in
  minimum_value/maximum_value/modulus/modular_value, so the many sites
  that parse those as integers are unaffected).
• expression_bounds.py: ///% introduce the flag when the divisor's
  range includes zero; additive, multiplicative, comparison (incl. &&/||),
  choice, and $max combinators propagate it.
• ir_util.py / header_generator.py: a value that can be undefined is not
  a compile-time constant even when its defined range collapses to a single
  value (e.g. 0 // d folds to {0}), so is_constant_type, constant_value,
  and the codegen literal-folding shortcut all decline to fold it.
• constraints.py: narrow the divisor check to provably-always-zero.
• emboss_arithmetic.h: rewrite FlooringQuotient / FlooringRemainder as
  hand-written "special ops" (like And/Or/Choice) that return an Unknown Maybe
  when the divisor is zero, even with both operands Known.
• Soundness guard for the Ok()/switch discriminant optimization: an
  existence condition that can be undefined is never treated as provably known.
  A differential test (FieldGatedByDivision) asserts the runtime Ok()
  behavior for a zero divisor.
• Tests + regenerated golden.

IsComplete() still returns false for a /0 size here; part 3 makes it return
true (the design doc's preferred behavior).

bazel test //... is green.

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PR chain (merge in order): #273 → #274 (this) → #275