feat(610): generated where-did-it-go index on each stripped file

Live docs point into the corpus BY DATE -- "see `decisions.md` 2026-07-10" --
about 32 such references across 12 files, plus the same form in historical issue
comments. The split would dangle every one of them.

Each stripped file now ends with a generated "Records formerly in this file"
index: date, title, and a link to the record's new path. A reader following a
date pointer lands on the file it names and resolves from there. That is far
cheaper and less error-prone than rewriting 32 references by hand, and it also
covers the issue-comment breadcrumbs, which cannot be rewritten at all.

Caught while verifying it: the generated `## Records formerly in this file`
heading is itself an H2, so the record parser counted one legacy-unmigrated
record per stripped file -- the notice went 0 -> 6. Same treatment as the
existing `## Index` section: skip it by name. SKIP_HEADINGS moved to
decisions_lib as the single source of truth, since three modules now need it.

Found by reading the validator's notice output on a trial migration, not by
inspection -- the corpus still validated OK, so nothing else would have flagged it.

scripts/migrate_decisions_split.py

@@ -93,6 +93,34 @@ def _rationale_body(rec: dl.Record) -> str:
     return "\n".join(lines).strip()
 
 
+_MOVED_HEADING = "## Records formerly in this file"
+
+
+def _moved_index(records: list[dl.Record]) -> str:
+    """A generated 'where did it go' index appended to each stripped file.
+
+    Live docs point into the corpus BY DATE — "see `decisions.md` 2026-07-10" — roughly 32 such
+    references across 12 files. The split would dangle every one of them. This index keeps them
+    resolvable: the reader lands on the file the pointer names and finds the date, the title, and
+    the record's new path. Cheaper and far less error-prone than rewriting 32 references by hand,
+    and it degrades gracefully for the historical issue comments that use the same form.
+    """
+    if not records:
+        return ""
+    lines = [
+        _MOVED_HEADING,
+        "",
+        "Each record below moved to its own file under `records/` (ersatztv#610); the rationale is",
+        "unchanged. Resolve by **key** — that is the stable identity. A date-based pointer from",
+        "another doc or an old issue comment should land here and then follow the link.",
+        "",
+    ]
+    for r in sorted(records, key=lambda x: (x.heading, x.key or "")):
+        dest = record_path(r).relative_to(dl.TOPIC_DIR)
+        lines.append(f"- {r.heading} — [`{r.key}`]({dest})")
+    return "\n".join(lines)
+
+
 def _preamble(path: Path, records: list[dl.Record]) -> str:
     """Everything in `path` that is not inside a record — kept, never deleted."""
     text = path.read_text(encoding="utf-8")
@@ -117,13 +145,15 @@ def plan() -> tuple[list[tuple[dl.Record, Path]], dict[Path, str]]:
     for src in sources:
         if dl.RECORDS_DIR in src.parents or dl.has_frontmatter(src.read_text(encoding="utf-8")):
             continue  # already migrated
-        recs = [r for r in dl.parse_file(src) if r.heading not in ("Index", "Active catalog", "Contents")]
+        recs = [r for r in dl.parse_file(src) if r.heading not in dl.SKIP_HEADINGS]
         keyed = [r for r in recs if r.key]
         if len(keyed) != len(recs):
             unkeyed = [r.heading for r in recs if not r.key]
             raise SystemExit(f"{src}: {len(unkeyed)} record(s) without a key, cannot file by key: {unkeyed}")
         moves += [(r, record_path(r)) for r in keyed]
-        preambles[src] = _preamble(src, recs)
+        body = _preamble(src, recs)
+        idx = _moved_index(keyed)
+        preambles[src] = f"{body.rstrip()}\n\n{idx}" if idx else body
     return moves, preambles