Phase 7.10: buxc2 now compiles multi-statement Bux programs

Parser (src_bux/parser.bux):
- Block stores statements as linked list (firstStmt/lastStmt/nextStmt)
- Block struct: added firstStmt, lastStmt fields
- Stmt struct: added nextStmt field

HIR Lowering (src_bux/hir_lower.bux):
- Lcx_LowerBlock: iterates statement linked list, chains HirNodes via child3
- hIf: else block stored in extraData (child3 reserved for block chaining)

C Backend (src_bux/c_backend.bux):
- hBlock: emits statements via child3 linked list with proper indentation
- hStore: combines alloca + value into single declaration (int x = value;)
- hIf: full if/else emission with proper newlines
- Function decl: proper return types and parameter types (not just "int")
- int main() wrapper: generated when Main function exists
- No duplicate return 0 when body already has return

Tested:
- buxc2 compiles hello world program → runs correctly
- buxc2 compiles multi-statement program (let, if, function calls) → runs correctly
- All 18 examples still pass, all unit tests pass

src_bux/ast.bux

@@ -127,6 +127,8 @@ struct Block {
     line: uint32,
     column: uint32,
     stmtCount: int,
+    firstStmt: *Stmt,
+    lastStmt: *Stmt,
 }
 
 // ---------------------------------------------------------------------------
@@ -170,6 +172,8 @@ struct Stmt {
     refStmtElse: *Block,     // else block
     // Else-if chain
     elseIfCount: int,
+    // Linked list
+    nextStmt: *Stmt,
 }
 
 // ---------------------------------------------------------------------------