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.

Makefile

@@ -5,7 +5,7 @@ BUILD_DIR := build
 # Project-local nimcache so CI can cache compiles (default is ~/.cache/nim).
 NIMFLAGS ?= --nimcache:nimcache
 
-EXAMPLES := hello fibonacci factorial structs enums methods algebraic_enums generics generics_struct generic_infer generic_infer2 extend_generic pattern_matching strings strings2 map result_option try_operator ownership ownership_checked ownership_release drop_early_return lifetime_elision ctfe ctfe_crc async concurrency os_time process json iter trait_bounds channel sync jwt stdlib_ergonomics tuples func_ptr map_remove array_iter_extra string_extra multi_closure iter_hof closure_control match_let string_interp iter_generic generic_infer_hof struct_tuple_pat match_block nested_patterns match_guards pattern_shadow move_field move_field_partial move_field_remaining move_field_nested move_field_ptr move_cross_fn c_precedence macro_twice macro_repeat macro_nested macro_hygiene macro_unhygienic macro_stmt_pat macro_tt macro_tt_raw macro_type collections_extra
+EXAMPLES := hello fibonacci factorial structs enums methods algebraic_enums generics generics_struct generic_infer generic_infer2 extend_generic pattern_matching strings strings2 map result_option try_operator ownership ownership_checked ownership_release drop_early_return lifetime_elision ctfe ctfe_crc async concurrency os_time process json iter trait_bounds channel sync jwt stdlib_ergonomics tuples func_ptr map_remove array_iter_extra string_extra multi_closure iter_hof closure_control match_let string_interp iter_generic generic_infer_hof struct_tuple_pat match_block nested_patterns match_guards pattern_shadow move_field move_field_partial move_field_remaining move_field_nested move_field_ptr move_cross_fn c_precedence macro_twice macro_repeat macro_nested macro_hygiene macro_unhygienic macro_stmt_pat macro_tt macro_tt_raw macro_type collections_extra generic_enum
 
 # Platform smoke (macOS CI): full EXAMPLES still runs on Linux.
 EXAMPLES_SMOKE := hello ownership ownership_release strings map move_field move_field_partial move_field_remaining move_field_nested move_field_ptr move_cross_fn c_precedence macro_twice macro_repeat macro_nested macro_hygiene macro_unhygienic macro_stmt_pat macro_tt macro_tt_raw ctfe_crc