backend: decide the seam between sentences instead of gluing them

A paragraph is its sentences concatenated. Nothing was put between them, which
is right for Chinese — 「。」 already separates — and wrong for English, where
`Adaptive capacity rises.` followed by `Relocation follows.` printed as
`...rises.Relocation...`: a sentence boundary the reader cannot see. CJK output
hid it, so it would have surfaced as an English bug later rather than now.

`sentence_separator` now returns "" or a single space per seam, and the document
carries it as `SentenceRead.separator_before`. The rule is about the seam, not
the language: a space goes in unless both sides are CJK. Mixed seams take the
space. Empty sentences take none. It is derived on every read and never stored,
so it cannot drift from the text, and the client prints `separator_before +
content` and adds no spacing of its own.

No splitting was added anywhere, and none exists: a sentence is one line in the
editor and nothing parses it. The single split this project has ever performed
was the one-time move of the old abstract column, which cut after 「。」 only —
conservative on purpose, since an English abstract is better left in one row
than cut at the first `et al.`.

Five smoke checks cover the seam (Chinese, English, the assembled paragraph,
and that no separator is stored inside the content). 45 checks pass.

backend/scripts/smoke_papers.py

@@ -183,7 +183,49 @@ def main() -> int:
             {"sentences": []},
         )
 
-        print("6. reject a citation with no quote")
+        print("6. the seam between sentences depends on the language")
+        # Chinese needs nothing after 。; English needs a space, or two
+        # sentences print as `First.Second.`
+        mixed = call(
+            "PUT",
+            f"/papers/{paper['id']}/paragraphs/{second_sort}",
+            {
+                "sentences": [
+                    {"content": "第一句。"},
+                    {"content": "第二句。"},
+                    {"content": "This is English."},
+                    {"content": "So is this."},
+                ]
+            },
+        )
+        paragraph_mixed = next(
+            p
+            for p in mixed["paragraphs"]
+            if p["paper_template_filed_sort"] == second_sort
+        )
+        separators = [s["separator_before"] for s in paragraph_mixed["sentences"]]
+        check("no space between Chinese sentences", separators[:2] == ["", ""], str(separators))
+        check("a space before an English sentence", separators[2] == " ", str(separators))
+        check("a space between English sentences", separators[3] == " ", str(separators))
+        assembled = "".join(
+            s["separator_before"] + s["content"] for s in paragraph_mixed["sentences"]
+        )
+        check(
+            "the assembled paragraph reads correctly",
+            assembled == "第一句。第二句。 This is English. So is this.",
+            assembled,
+        )
+        check(
+            "the separator is not part of the stored text",
+            all(
+                not s["content"].startswith(" ") and not s["content"].endswith(" ")
+                for s in paragraph_mixed["sentences"]
+            ),
+        )
+        # Clear it again so the counts asserted below stay meaningful.
+        call("PUT", f"/papers/{paper['id']}/paragraphs/{second_sort}", {"sentences": []})
+
+        print("7. reject a citation with no quote")
         bad = call(
             "PUT",
             f"/papers/{paper['id']}/paragraphs/{first_sort}",
@@ -192,7 +234,7 @@ def main() -> int:
         )
         check("citation without content refused", "detail" in bad)
 
-        print("7. a sentence with no paragraph in the template survives a switch")
+        print("8. a sentence with no paragraph in the template survives a switch")
         # A position neither template defines, so the check means the same
         # thing before and after the switch below.
         orphan_sort = (
@@ -216,7 +258,7 @@ def main() -> int:
             p for p in document["paragraphs"] if p["paper_template_filed_sort"] == orphan_sort
         )["name"] is None)
 
-        print("8. switch the template")
+        print("9. switch the template")
         switched = call("PATCH", f"/papers/{paper['id']}", {"template_id": narrow["id"]})
         check("template changed", switched["template_id"] == narrow["id"])
         document = call("GET", f"/papers/{paper['id']}/document")
@@ -246,7 +288,7 @@ def main() -> int:
             ),
         )
 
-        print("9. move one paragraph onto another position")
+        print("10. move one paragraph onto another position")
         document = call(
             "PUT",
             f"/papers/{paper['id']}/paragraphs/{orphan_sort}",
@@ -261,18 +303,18 @@ def main() -> int:
             all(p["paper_template_filed_sort"] != orphan_sort for p in document["paragraphs"]),
         )
 
-        print("10. paper list reports progress")
+        print("11. paper list reports progress")
         listing = call("GET", f"/papers?keyword=冒烟&status=writing")
         check("found by keyword and status", listing["total"] == 1, str(listing["total"]))
         row = listing["items"][0]
         check("paragraph count", row["paragraph_count"] == 1, str(row["paragraph_count"]))
         check("sentence count", row["sentence_count"] == 3, str(row["sentence_count"]))
 
-        print("11. a template in use cannot be deleted")
+        print("12. a template in use cannot be deleted")
         conflict = call("DELETE", f"/templates/{narrow['id']}", expect=409)
         check("template delete refused", "正被论文使用" in conflict["detail"], conflict["detail"])
 
-        print("12. single sentence edit and delete")
+        print("13. single sentence edit and delete")
         sentence_id = target["sentences"][0]["id"]
         updated = call(
             "PATCH",
@@ -288,7 +330,7 @@ def main() -> int:
             sum(len(p["sentences"]) for p in document["paragraphs"]) == 2,
         )
 
-        print("13. a paper with no template still works")
+        print("14. a paper with no template still works")
         bare = call(
             "POST",
             "/papers",
@@ -311,7 +353,7 @@ def main() -> int:
         call("DELETE", f"/papers/{bare['id']}", expect=204)
         created_papers.remove(bare["id"])
 
-        print("14. delete the paper")
+        print("15. delete the paper")
         call("DELETE", f"/papers/{paper['id']}", expect=204)
         created_papers.clear()
         check(