Add conditional expressions, truthy coalescing, compound assignment and discard

[Leitwolf11]

Summary

Adds four Spec forms that the compiler did not accept (Spec/Language/Operators.vxs §15–18, Decls.vxs §33):

• condition ? first : second — exactly one result is evaluated.
• left ?: fallback (also left ? : fallback) — keeps a left value that is true in Boolean context; the left operand is evaluated once.
• Compound assignment statements += -= *= /= //= %= **= <<= >>= &= ^= |=, including in for update lists.
• The discard statement _ = value;.

?? and ??= are now lexed as their own tokens and rejected with VXP0030 / VXP0031 until nullable types exist.

How it works

• Frontend: four new AST nodes carried through every pass. Wildcard fallbacks that would have silently treated a conditional as call-free were replaced by explicit cases.
• Core: a lazy CoreConditional expression in the Haskell and native models, both verifiers and both wire codecs. Core wire is now v8 (expression tag 6). CorePrep, Xpp and Xmm are unchanged: the expression becomes control flow before CorePrep.
• Optimizer: effect inference, integer facts, constant folding, liveness and inlining treat the arms as alternative paths.
• CorePrep: a branch over one result slot, seeded before the branch and assigned in each arm block — the same pattern as short-circuit operators.
• The native adapter now binds a let value with its own operation, matching the Haskell adapter. The extra temporary made the two adapters disagree for every non-atomic value, which ?: would have exposed.

Limits

• Conditional results are bool or numeric only (VXT0039, VXC1070). A slot holding an owned value needs move and release rules the backend does not have.
• Assignment is still a statement with a named target; a = b = 10 and (value += 5) are not implemented.
• Known and unchanged: the Haskell adapter spells the zero of a floating condition comparison as an integer payload, the native one as a floating literal. An existing test pins the Haskell form; it is not touched here.

Evidence

• 115 new Haskell cases; native CorePrep-shape, verifier and wire tests; JIT execution tests under both optimizer settings whose unselected arm divides by zero or recurses without bound.
• The differential generator has three new program shapes with host-computed expectations; the smoke program runs 216 cases.
• Local (Windows): cabal compiler and fuzz-feedback suites, develop test (21 native suites), verify-docs, verify-examples, clang-format, git diff --check — all pass.
• Not run locally: ASan/UBSan suites, libFuzzer campaigns, macOS and Linux.