feat: generic enums, fix is-operator, Type_Eq, hardcoded limit diagnostics

- feat: generic enum support (parser, lowering, codegen — both selfhost + bootstrap)
  - enum Result<T,E> { Ok(T), Err(E) } parsing + monomorphization
  - tag constant mangling in monomorphized function bodies
  - data field access (l-value + r-value) for generated enum instances
  - multiple concrete instances in same file
  - HIR walker for enum reference mangling (selfhost + bootstrap)

- feat: stdlib Result<T,E> and Option<T> made truly generic
  - breaking: explicit type args required (Result<int, String>)

- fix: 'is' operator — lowering to hBinary tag comparison + C backend fallback
- fix: Type_Eq structural comparison (inner types for pointer/slice/tuple)
- fix: hardcoded limit diagnostics (>8 params/variants/captures now emit errors)
- docs: Iter<T> safety warning for dangling pointer
- docs: IMPROVEMENTS.md — comprehensive plan and changelog
- test: generic_enum example added to EXAMPLES

All tests pass (0 FAIL). Selfhost loop deterministic.

lib/Iter.bux

@@ -2,6 +2,10 @@ module Std::Iter {
 
     import Std::Array::*;
 
+    // SAFETY: Iter<T> stores a raw *T pointer to the source array's element buffer.
+    // The iterator MUST NOT outlive the source Array<T>. Modifying the source array
+    // (e.g. Array_Push which may reallocate the buffer) while an iterator is active
+    // will result in a dangling pointer and undefined behavior.
     struct Iter<T> {
         data: *T,
         len: uint,