DD-50 Rule 2: replace runtime-throw stand-in with analysis-pass diagnostic

[oubiwann]

Context

DD-50 introduced Rule 2: if in expression position with no else
branch is a compile error. Example:

(bind x (if cond value))   ;; should be a compile error per Rule 2

The current implementation in both compilers emits a runtime
throw new TypeError("COMPILE_ERROR: ...") as an MVP
stand-in. The throw fires at runtime if the path is ever executed,
which is semantically correct (the program is broken) but
architecturally imprecise — DD-50's spec called for a true
compile-time error.

What needs to change

Replace the runtime-throw stand-in with a true compile-time
diagnostic emitted via the existing diagnostic channel before
emit, modelled on:

• Rust: crates/lykn-lang/src/analysis/match_check.rs
  (exhaustiveness check pattern) or
  crates/lykn-lang/src/analysis/scope.rs (collision detection
  from DD-49 Rule 6).
• JS: equivalent diagnostic-emitting path in the JS surface
  expander.

After this change, the existing test that asserts on the
COMPILE_ERROR throw output should still pass (or be updated to
assert on the analysis-pass diagnostic instead).

Files

• crates/lykn-lang/src/emitter/forms.rs — emit_if_expression
  and convert_to_expression (search for COMPILE_ERROR:).
• packages/lang/surface.js — search for COMPILE_ERROR: and
  the if-handler's no-else error path.
• crates/lykn-lang/src/analysis/ — natural home for the new
  analysis pass.

Scope

Medium. Self-contained refactor; existing test coverage from
DD-50.5 and DD-50.5 addendum protects regression.

References

• DD-50: docs/design/05-active/0050-position-aware-compilation-of-conditional-and-block-forms.md Rule 2
• M7 closing report fast-follow Lock-file design for lykn projects #7