docs: scrub remaining stale AST-codegen references

Fallout from removing the `--use-ast` flag and the AST codegen.
Four references in `docs/future-work.md` and one in `plan.md` were
still implying a parallel AST code path existed:

- `future-work.md` "--debug CLI flag wired" entry — said the flag
  threads through `CodeGen::with_debug` (the old AST codegen's
  builder). Updated to `IrCodeGen::with_debug`.
- `future-work.md` IR debug.log/assert entry — had a parenthetical
  "(same behavior as AST codegen)" that no longer makes sense
  without an AST codegen to compare against. Dropped.
- `future-work.md` inline assembly entry — said "Both IR and AST
  codegen splice parsed instructions directly into the output
  stream." There's only the IR codegen now. Updated.
- `future-work.md` `on scanline(N)` entry — described scanline
  handling as "codegen (MMC3 IRQ vector wiring) is still TODO."
  That wiring has been in place for a while: the IR codegen
  emits `__irq_user` + `__ir_mmc3_reload` with per-state
  dispatch, and `examples/scanline_split.ne` and
  `examples/mmc3_per_state_split.ne` both exercise it in the
  jsnes smoke test. Rewrote the entry to match reality.
- `plan.md` M2 "Compiler phases built" list — had "Codegen from
  IR (replacing direct AST codegen)." The present-tense
  parenthetical read as if AST codegen still existed; stripped
  it. M1's "direct AST → 6502, skip IR for this milestone" line
  is left as a historical milestone scope description.

One intentional mention remains: `future-work.md` "Recently
completed" bullet explicitly notes the AST-based path and the
`--use-ast` flag were removed. That's the correct tombstone.

https://claude.ai/code/session_014Z5y3Q9krLcAxYpZQJhZ5V

plan.md

@@ -932,7 +932,7 @@ start Main
 - Full analyzer (call graph, depth limits, recursion detection, state validation)
 - IR lowering
 - Basic optimizer (constant folding, dead code)
-- Codegen from IR (replacing direct AST codegen)
+- Codegen from IR
 - Memory map report
 - Call graph report