fix(docs): align MODULE-SYSTEM-PROGRESS with main — clears #614 conflict (#616)

## What

Clears the **last** merge conflict blocking #614. After #615 was
**squash-merged**, the feature branch gained `main`'s content but not
its ancestry, so #614's 3-way merge re-derived one conflict:
`docs/history/MODULE-SYSTEM-PROGRESS` (modified on `main` in #602 /
migrated to `.adoc` + deleted on this branch).

## Fix

Match `main` exactly for that one file — restore `main`'s `.md`, drop
the `.adoc`. **No content is lost**: `main`'s `.md` already carries the
#138 codegen-follow-up note. The `.md`→`.adoc` migration can be redone
later as a standalone DOC-FORMAT change.

This resolution is **squash-robust** — it makes the file
modify/modify-identical rather than modify/delete, so #614 stays
conflict-free regardless of how this PR is merged.

## Verified

- Trial merge of the fixed feature branch into `main`: **0 conflicts**
("Automatic merge went well").
- `dune build` clean (docs-only delta from the already-green #615 tree —
`dune test` 534/534 there).

Merging this into `feat/solo-core-metatheory-proofs` makes **#614
mergeable** and triggers its full CI.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01Lz7pRcec2Z3tVtaAhvB3M8

---
_Generated by [Claude
Code](https://claude.ai/code/session_01Lz7pRcec2Z3tVtaAhvB3M8)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath