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.

Makefile

@@ -3,7 +3,7 @@ SRC := src/main.nim
 OUT := buxc
 BUILD_DIR := build
 
-EXAMPLES := hello fibonacci factorial structs enums methods algebraic_enums generics pattern_matching strings map result_option try_operator generics_struct
+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
 
 .PHONY: all build dev test clean test-examples
 
@@ -53,3 +53,12 @@ clean:
 	rm -rf nimcache
 	rm -rf examples_pkg
 	rm -rf _test_tmp_pkg
+
+selfhost: build
+	@echo "=== Phase 7.9: Building self-hosted compiler ==="
+	@rm -rf _selfhost/src
+	@mkdir -p _selfhost/src
+	@cp src_bux/*.bux _selfhost/src/
+	@cp _selfhost/src/main.bux _selfhost/src/Main.bux 2>/dev/null || true
+	@cd _selfhost && ../$(OUT) build
+	@echo "=== Self-hosted compiler built successfully ==="