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/token.nim

@@ -50,6 +50,7 @@ type
     tkSuper           # super
     tkSizeOf          # sizeof
     tkOwn             # own (gradual ownership transfer)
+    tkDiscard         # discard (evaluate and throw away)
 
     ##Punctuation
     tkLParen          # (
@@ -141,7 +142,7 @@ proc isKeyword*(kind: TokenKind): bool =
      tkBreak, tkContinue, tkReturn, tkMatch,
      tkFunc, tkLet, tkVar, tkConst, tkType, tkStruct, tkEnum,
      tkUnion, tkInterface, tkExtend, tkModule, tkImport,
-      tkPub, tkExtern, tkAs, tkIs, tkNull, tkSelf, tkSuper, tkOwn:
+      tkPub, tkExtern, tkAs, tkIs, tkNull, tkSelf, tkSuper, tkOwn, tkDiscard:
     true
   else:
     false
@@ -196,6 +197,7 @@ proc keywordKind*(text: string): TokenKind =
   of "super": tkSuper
   of "sizeof": tkSizeOf
   of "own": tkOwn
+  of "discard": tkDiscard
   of "true", "false": tkBoolLiteral
   else: tkIdent
 
@@ -240,6 +242,7 @@ proc tokenKindName*(kind: TokenKind): string =
   of tkSelf: "'self'"
   of tkSuper: "'super'"
   of tkOwn: "'own'"
+  of tkDiscard: "'discard'"
   of tkLParen: "'('"
   of tkRParen: "')'"
   of tkLBrace: "'{'"