Phase 7.9: Self-hosted compiler buxc2 builds and runs! 🎉

buxc (Nim bootstrap) successfully compiles buxc2 (Bux self-hosted compiler)
into a working 88KB ELF x86-64 binary.

Compiler fixes (Nim):
- duplicate symbol: user funcs shadow stdlib funcs via mergeDecls()
- forward declarations: func without body + definition (both orderings)
- extern func dedup: same extern in multiple files
- discard keyword: new language keyword, lowered to expr stmt or no-op
- parser: keywords as field names + advance-on-error safeguard
- parser: var without initializer (zero-init)
- parser: multi-line || && continuation expressions
- parser: else-if chain newline handling
- C backend: const declarations emitted as #define
- C backend: load(field_ptr) → base.field optimization (fixes lvalue errors)

Source fixes (src_bux/*.bux):
- types.bux: tk* → ty* prefix for type kind constants (avoid token conflict)
- sema.bux: remaining tk* → ty* references, StringMap → *void workaround
- hir_lower.bux: ekReturn removed, Lcx_LowerParam helper, *f dereference
- c_backend.bux: &mod.funcs[i] for pointer passing
- cli.bux: ReadFile/WriteFile → bux_read_file/bux_write_file wrappers
- parser.bux: pathStr.len → String_Len(pathStr)
- types.bux: "*" + x → String_Concat("*", x)

Build system:
- Makefile: added 4 missing examples + selfhost target
- PLAN.md: updated with actual project state, Phase 7.9 marked complete

All 18 examples pass, all unit tests pass.

src_bux/cli.bux

@@ -4,8 +4,16 @@ module Cli {
 
 extern func PrintLine(s: String);
 extern func Print(s: String);
-extern func ReadFile(path: String) -> String;
-extern func WriteFile(path: String, content: String) -> bool;
+extern func bux_read_file(path: String) -> String;
+extern func bux_write_file(path: String, content: String) -> bool;
+
+func ReadFile(path: String) -> String {
+    return bux_read_file(path);
+}
+
+func WriteFile(path: String, content: String) -> bool {
+    return bux_write_file(path, content);
+}
 
 // Import the compiler pipeline
 // In self-hosting mode, these are compiled together from src_bux/