emrg: the open-source template edits a document with its translated twin

emrg/server/open_source_prompt.md

@@ -244,6 +244,7 @@ cd "$DEV" && git checkout -b <branch name per project convention> "upstream/$DEF
 - **Work in the clone from §3.1.4** (`cd "$DEV"`): every edit lands there, and `{{ source_dir }}` stays a read-only reference.
 - **One problem per change** — no opportunistic refactoring; the change should follow the conventions read in §3.1.3 and the design intent from §3.1.3.
 - **Read the surrounding context before editing**: understand the responsibilities of the code you touch.
+- **A document with a translated twin is edited in one change.** Where the file you are changing has a counterpart in another language, both are changed in the same change and read against each other before it is committed: a twin left behind states the old rule to the next reader, and the reader who opens that side first is told the opposite of what the project now does. Where the project states which copy governs a disagreement, that statement sets the order — the other copy follows it, never the reverse.
 
 ##### 3.1.6 Test
 

tests/test_prompt_templates.py

@@ -2134,6 +2134,78 @@ def test_the_published_contribution_pin_answers_both_ways():
         )
 
 
+# ── §3.1.5 lists what an edit must satisfy, and it said nothing about a two-language file ────
+#
+# A project that keeps a translated copy of a document has two readers of one instruction, and a
+# change that updates only the side its round was reading leaves the other side stating the old
+# rule — silently, because no suite in the target repository compares the two. The rule is pinned
+# by **position**, not by presence: `_heading_above` reads `##`-to-`####`, and §3.1.5 is a
+# five-level heading, so the same sentence under §3.1.6 would be read here as a testing rule and
+# under §3.1.1 as an entry check.
+_OPEN_SOURCE_TWIN_PHRASE = "A document with a translated twin is edited in one change"
+_OPEN_SOURCE_TWIN_SECTION = "##### 3.1.5 Implement"
+_OPEN_SOURCE_TWIN_NEXT = "##### 3.1.6 Test"
+
+
+def _fifth_level_section_of(text: str, phrase: str) -> str | None:
+    """The nearest `#####` heading before `phrase`, or `None` when the phrase is absent."""
+    heading = None
+    for line in text.splitlines():
+        if line.startswith("##### "):
+            heading = line.strip()
+        if phrase in line:
+            return heading
+    return None
+
+
+def test_the_open_source_prompt_pairs_a_document_with_its_translated_twin() -> None:
+    """An edit rule that omits the twin is an edit rule half the projects cannot follow.
+
+    The English-only principle at the top of the template does not cover this: it governs what
+    the task sends outward, not the repository's own documents, and it says nothing about a copy
+    a change leaves behind.
+    """
+    text = (PROMPTS_DIR / "open_source_prompt.md").read_text(encoding="utf-8")
+
+    assert text.count(_OPEN_SOURCE_TWIN_PHRASE) == 1, (
+        f"{_OPEN_SOURCE_TWIN_PHRASE!r} appears {text.count(_OPEN_SOURCE_TWIN_PHRASE)} times — "
+        "a rule written twice is two answers that will disagree"
+    )
+    for heading in (_OPEN_SOURCE_TWIN_SECTION, _OPEN_SOURCE_TWIN_NEXT):
+        assert f"\n{heading}" in "\n" + text, (
+            f"the section this pin reads position against is gone: {heading!r} — a positional "
+            "reading of a missing bound compares against nothing"
+        )
+    assert _fifth_level_section_of(text, _OPEN_SOURCE_TWIN_PHRASE) == _OPEN_SOURCE_TWIN_SECTION, (
+        "the translated-twin rule is no longer under "
+        f"{_OPEN_SOURCE_TWIN_SECTION!r} (found: "
+        f"{_fifth_level_section_of(text, _OPEN_SOURCE_TWIN_PHRASE)!r}) — a rule in a section "
+        "that does not own its subject is one a round reads as something else"
+    )
+
+
+def test_the_translated_twin_pin_reads_position_and_absence() -> None:
+    """The controls: a deleted rule and a moved rule must each redden, and they are not the same
+    arm — deletion is what a rewrite does, relocation is what a busy edit does."""
+    text = (PROMPTS_DIR / "open_source_prompt.md").read_text(encoding="utf-8")
+
+    deleted = text.replace(_OPEN_SOURCE_TWIN_PHRASE, "")
+    assert deleted != text, (
+        f"{_OPEN_SOURCE_TWIN_PHRASE!r} is pinned but absent, so this arm measured nothing"
+    )
+    assert _fifth_level_section_of(deleted, _OPEN_SOURCE_TWIN_PHRASE) is None, (
+        "a deleted rule still reads as present, so the pin would not notice its own rule going"
+    )
+
+    moved = deleted.rstrip() + f"\n\n{_OPEN_SOURCE_TWIN_PHRASE}\n"
+    found = _fifth_level_section_of(moved, _OPEN_SOURCE_TWIN_PHRASE)
+    assert found is not None, "the relocation arm lost the phrase it moved, not the position"
+    assert found != _OPEN_SOURCE_TWIN_SECTION, (
+        f"moving the phrase to the end of the file left it reading as {found!r} — the pin is "
+        "reading presence, not position"
+    )
+
+
 # --------------------------------------------------------------------------- the inbound half
 #
 # A round of the promotion task is written as four outward steps: find a topic, join it,