# Step 2.1 — Image extension `.MCD` → `.MC4` (tag: `step2.1`)

Commit `5c56c9f` — "step2.1: rename 64-bit images .MCD -> .MC4 per
spec sections 8 and 14" (57 files, pure rename + 8 one-line edits).

## Rationale

The spec already mandates `.MC4` (§8 file format, §14 migration
row), and the tree was inconsistent (`boot.mc4` vs `example.MCD`).
The rename also disambiguates formats: legacy 16-bit inputs stay
`.MCD`, 64-bit outputs become `.MC4`, so the `trans8to64` flow
(`in.MCD` → `out.MC4`) is self-documenting.

## What changed

- Producers: `mkdemo`→`boot.MC4`, `mkdtest`→`example.MC4`,
  `mkread`→`readtest.MC4`, `mkdep`→`STAK/DEP.MC4`,
  `MGen` emits `<Mod>.MC4`, `trans8to64` usage (input side kept
  `.MCD`).
- `Loader2` dependency suffix, `MC64.mod` boot name, `mcint`
  usage text (all `.MC4`, uppercase per spec).
- Docs (surgical): `SESSION.md`, `session-summary.md`, spec §16
  boot note. Historical `.MCD` mentions kept on purpose (16-bit
  doc, §14 migration row, translator input side).
- Tracked binaries renamed via `git mv` + full rebuild.

## Verification after the rename

- mc64: `example.MC4` 24/24 exit 0, `boot.MC4` → Hello,
  `readtest.MC4` stdin echo, `mcint DEP.MC4` → cross-module ok
  (proves the loader suffix change), `mc64` → `[vm end]`.
- m2c: 25/25, `.MC4` files regenerated by the suite.