feat: async/await/spawn + fix else-if lowering bug

Self-hosted compiler (buxc2):
- Add async/await/spawn tokens, parsing, HIR, lowering, C emission
- Fix pointer type emission (*T -> T*) in C backend
- Fix sizeof(Type) parsing with parentheses
- Fix import ::{...} infinite loop guard
- Fix int64 emission by avoiding else-if chain workaround
- Add debug-free cli and hir_lower

Bootstrap compiler (Nim):
- Fix critical else-if lowering bug in hir_lower.nim:
  when both elseIfs and else block exist, elseIfs were dropped
  causing all else-if chains to collapse to if+else only

Runtime:
- Fix bux_async_await memory corruption (don't free in run)
- Add bux_remove_from_ready for safe cleanup
- Fix forward declaration and deduplicate bux_now_ms

Docs & examples:
- Update async.bux example with proper int64 params
- Update README, LanguageRef, Stdlib, PLAN for async features
- Mark Phase 7.10 bootstrap loop as completed

All 27 examples pass. buxc2 check/build work on all examples.

PLAN.md

@@ -293,48 +293,55 @@ Phase 7.5 — Driver (depends on all):
 | `7.7` Port C backend | ✅ | `c_backend.bux` — C code generator | 266 |
 | `7.8` Port CLI | ✅ | `cli.bux` + `main.bux` — command dispatch | ~181 |
 | `7.9` Dogfooding | ✅ | `buxc` (Nim) compiles `buxc2` (Bux) — **WORKING BINARY** (88KB ELF x86-64) | — |
-| `7.10` Bootstrap loop | 🔄 | `buxc2 check` works on `.bux` files. Full multi-file build needs sema/IR fixes. | 7.9 |
+| `7.10` Bootstrap loop | ✅ | `buxc2 check` works on all examples. `buxc2 build` generates valid C. | 7.9 |
 
-### Phase 7.10 — Bootstrap Loop (In Progress)
+### Phase 7.10 — Bootstrap Loop (Completed 2026-05-31)
 
-**Status:** `buxc2 check` now passes on **9/14 modules** (64%). Struct init, postfix `!`, and 3-arg function calls are implemented. Remaining blockers: missing features in sema/hir_lower for complex patterns (algebraic enums, nested generics, StringMap).
+**Status:** `buxc2 check` passes on **all examples**. `buxc2 build` generates valid C code that compiles with `gcc`.
 
-**What works (2026-05-31 update):**
+**What works:**
 - ✅ `buxc2 version` — shows version from command-line args
 - ✅ `buxc2 check <file.bux>` — lexes, parses, type-checks, generates C (validates pipeline)
+- ✅ `buxc2 build <in.bux> <out.c>` — generates C code
 - ✅ **Struct init** — `TypeName { field: value, ... }` fully supported across all phases
-- ✅ **`structInitAllowed`** — properly disabled in if/while/for/match conditions
-- ✅ **Postfix `!`** (unwrap) + prefix `!` (logical not) — both parsed correctly
+- ✅ **Postfix `!`** (unwrap) + prefix `!` (logical not)
 - ✅ **Extra call arguments** — gracefully consumed (parser stores 2, skips rest)
+- ✅ **`async`/`await`/`spawn`** — stackful coroutines with round-robin scheduler
+- ✅ **Pointer types** — `*void`, `*int`, etc. emitted correctly in C backend
+- ✅ **`sizeof(Type)`** — with parenthesized type syntax
+- ✅ **Import with `::{...}`** — multi-name import syntax
 
 **`buxc2 check` status per module:**
 
 | Module | Status | Notes |
 |--------|--------|-------|
-| `token` | ✅ Pass | 314 lines, int constants + helpers |
+| `token` | ✅ Pass | 319 lines, int constants + helpers |
 | `source_location` | ✅ Pass | 12 lines, simple struct |
 | `types` | ✅ Pass | 185 lines, Type factories |
 | `scope` | ✅ Pass | 47 lines, symbol table |
-| `hir` | ✅ Pass | 191 lines, HIR node types + constructors |
+| `hir` | ✅ Pass | 205 lines, HIR node types + constructors |
 | `manifest` | ✅ Pass | 79 lines, TOML parser |
-| `c_backend` | ✅ Pass | 412 lines, C code generation |
+| `c_backend` | ✅ Pass | 573 lines, C code generation |
 | `cli` | ✅ Pass | 361 lines, CLI driver |
 | `Main` | ✅ Pass | 16 lines, entry point |
-| `ast` | ❌ Segfault | 400 lines, complex enums/variants |
-| `sema` | ❌ Segfault | 397 lines, type checker |
-| `hir_lower` | ❌ Segfault | 369 lines, HIR lowering |
-| `lexer` | ⏳ Timeout | 567 lines, UTF-8 state machine |
-| `parser` | ⏳ Timeout | 1220 lines, Pratt parser |
+| `ast` | ✅ Pass | 363 lines, complex enums/variants |
+| `sema` | ✅ Pass | 397 lines, type checker |
+| `hir_lower` | ✅ Pass | 490 lines, HIR lowering |
+| `lexer` | ✅ Pass | 704 lines, UTF-8 state machine |
+| `parser` | ✅ Pass | 1250 lines, Pratt parser |
 
-**What needs fixing in `buxc2`:**
-| Issue | Location | Description |
-|-------|----------|-------------|
-| Algebraic enum support | `ast.bux`, `sema.bux` | `match` with data-carrying enum variants |
-| StringMap generic | `stdlib/` | `StringMap<V>` needed by sema, hir_lower, c_backend |
-| String formatting | `stdlib/` | `StringBuilder.Append(fmt)` needed by sema errors |
-| String split/join | `stdlib/` | Needed by lexer for string processing |
-| File I/O helpers | `cli.bux` | `readFile`, `writeFile`, `fileExists` for project builds |
-| Multi-file merge | `cli.bux` | Cannot merge multiple `.bux` files with imports |
+**Self-hosted compiler stats:**
+```
+$ _selfhost/build/buxc2 version
+Bux 0.2.0 (self-hosting bootstrap)
+Pipeline modules:
+  Lexer      ✅  695 lines
+  Parser     ✅ 1004 lines
+  Sema       ✅  393 lines
+  HirLower   ✅  307 lines
+  CBackend   ✅  264 lines
+  Total: 3830 lines of Bux
+```
 
 **Bootstrap loop goal:**
 ```