C compiler stage 5: local variables and the first semantic pass (task C5)

[prozak]

Add local variables -- declarations, assignment, references -- following blog
part 5. Two genuinely new things: a stack frame, and the compiler's first
semantic pass.

• c.pg: assignment enters the cascade at the top (lowest precedence,
  right-recursive). Its LHS is a bare Ident, which keeps the grammar
  LALR(1)-clean (an Ident is an assignment target only when '=' follows, else
  it reduces to a Factor -- no conflict) and makes a + 3 = 4 a syntax error,
  as the blog wants. Factor gains VarRef; Statement gains expression
  statements and the two declaration forms.
• Resolve.hs (new): the first semantic pass. Walks the statement list in
  order, assigns each local a stack offset, and rejects undeclared/
  use-before-declaration/duplicate-declaration with Either String. The
  "variables referenced anywhere in this node" query is one SYB listify over
  the derived Data instances rather than a hand recursion over the Exp
  cascade -- QQ for targeted matching, generic programming for whole-tree
  queries.
• Main.hs: runs resolve between parse and codegen; semantic errors reject
  with the same no-artifacts contract as parse errors.
• asm.pg: 64-bit movq/subq, %rbp/%rsp, a Mem operand (signed disp(base)), and
  signed num so -4(%rbp) lexes as one displacement.
• Codegen.hs: now ReaderT VarMap (State Int) -- Reader for the offset map,
  State for labels. Prologue/epilogue frame every function; references load
  and assignments store (the value stays in %eax, so assignment is an
  expression).
• TestQQ.hs: assignment (incl. right-assoc and value), VarRef, declaration,
  the Mem operand, and a parse->resolve->codegen->emit->parse round trip.
  tests/ gains variable programs and four semantic-error cases.

Verified: tutorial suite 88/88; official write_a_c_compiler stage 5 17/17,
stages 1-3 still green, and stage 4 rose 24 -> 27 (its skip_on_failure
short-circuit tests need variables, so they now compile and pass -- a second
check that stage 4's jumps are right). Adds tutorial/05-variables.md and
updates the index, status, layout, and roadmap.

https://claude.ai/code/session_01TXSeYgnZJZWC6dczN6d5E9