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/hir_lower.nim

@@ -773,7 +773,9 @@ proc lowerStmt(ctx: var LowerCtx, stmt: Stmt): HirNode =
     return ctx.flushPending(ctx.lowerExpr(stmt.stmtExpr))
 
   of skLet:
-    let initHir = ctx.lowerExpr(stmt.stmtLetInit)
+    var initHir: HirNode = nil
+    if stmt.stmtLetInit != nil:
+      initHir = ctx.lowerExpr(stmt.stmtLetInit)
     let allocaType = if stmt.stmtLetType != nil:
       case stmt.stmtLetType.kind
       of tekNamed:
@@ -785,16 +787,17 @@ proc lowerStmt(ctx: var LowerCtx, stmt: Stmt): HirNode =
         let elemType = ctx.resolveTypeExpr(stmt.stmtLetType.sliceElement)
         makeSlice(elemType)
       else: makeUnknown()
-    else:
+    elif stmt.stmtLetInit != nil:
       ctx.resolveExprType(stmt.stmtLetInit)
+    else:
+      makeUnknown()
 
     let alloca = hirAlloca(stmt.stmtLetName, allocaType, loc)
     let varNode = hirVar(stmt.stmtLetName, makePointer(allocaType), loc)
-    let store = hirStore(varNode, initHir, loc)
     # Track type expr for generic method inference
     if stmt.stmtLetType != nil:
       ctx.varTypeExprs[stmt.stmtLetName] = stmt.stmtLetType
-    elif stmt.stmtLetInit.kind == ekStructInit and stmt.stmtLetInit.exprStructInitTypeArgs.len > 0:
+    elif stmt.stmtLetInit != nil and stmt.stmtLetInit.kind == ekStructInit and stmt.stmtLetInit.exprStructInitTypeArgs.len > 0:
       ctx.varTypeExprs[stmt.stmtLetName] = TypeExpr(
         kind: tekNamed,
         loc: stmt.stmtLetInit.loc,
@@ -804,7 +807,9 @@ proc lowerStmt(ctx: var LowerCtx, stmt: Stmt): HirNode =
     var stmts = ctx.pendingStmts
     ctx.pendingStmts = @[]
     stmts.add(alloca)
-    stmts.add(store)
+    if initHir != nil:
+      let store = hirStore(varNode, initHir, loc)
+      stmts.add(store)
     return hirBlock(stmts, nil, makeVoid(), loc)
 
   of skReturn: