Show VERSIONS.md reason extracts in dwarf status

Parses the gen/schema log lines apply_bump() writes to VERSIONS.md and
prints a truncated reason under each gen/schemaN row, so `dwarf status`
answers "why does this version exist" without opening the changelog.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/bump_dataset_version.py

@@ -26,6 +26,17 @@ from pathlib import Path
 GEN_RE = re.compile(r"^gen(\d+)$")
 SCHEMA_RE = re.compile(r"^schema(\d+)$")
 
+# Match the log lines written by apply_bump()/plan_bump_gen()/plan_bump_schema():
+#   - `gen2` (kind=steps) — 2026-01-01 — reason text (by)
+#   - `gen2`/`schema2` (kind=steps) — 2026-01-01 — reason text (by)
+VERSIONS_SCHEMA_LINE_RE = re.compile(
+    r"^- `(?P<gen>gen\d+)`/`(?P<schema>schema\d+)` \(kind=(?P<kind>[\w-]+)\)"
+    r" — \d{4}-\d{2}-\d{2} — (?P<reason>.+)$"
+)
+VERSIONS_GEN_LINE_RE = re.compile(
+    r"^- `(?P<gen>gen\d+)` \(kind=(?P<kind>[\w-]+)\) — \d{4}-\d{2}-\d{2} — (?P<reason>.+)$"
+)
+
 # One color per tree level in `dwarf status` output, so the eye can jump
 # straight to e.g. "all the schema rows" or "all the totals".
 _LEVEL_COLORS = {
@@ -34,6 +45,7 @@ _LEVEL_COLORS = {
     "bucket": "\033[34m",  # blue — raw/processed subtotals
     "schema": "\033[32m",  # green — schemaN rows
     "root": "\033[1;35m",  # bold magenta — derived/, pools/, grand total
+    "reason": "\033[2m",  # dim — VERSIONS.md reason extract under gen/schema rows
 }
 _RESET = "\033[0m"
 
@@ -232,6 +244,35 @@ def _referenced_parquet_count(
     return total, referenced
 
 
+def _parse_versions(
+    versions_path: Path,
+) -> tuple[dict[tuple[str, str], str], dict[tuple[str, str, str], str]]:
+    """Read VERSIONS.md and return (gen_reasons, schema_reasons) keyed by
+    (kind, gen_tag) and (kind, gen_tag, schema_tag) respectively. Later entries
+    for the same key win, since VERSIONS.md is append-only and chronological."""
+    gen_reasons: dict[tuple[str, str], str] = {}
+    schema_reasons: dict[tuple[str, str, str], str] = {}
+    if not versions_path.is_file():
+        return gen_reasons, schema_reasons
+    for line in versions_path.read_text().splitlines():
+        line = line.strip()
+        m = VERSIONS_SCHEMA_LINE_RE.match(line)
+        if m:
+            schema_reasons[(m["kind"], m["gen"], m["schema"])] = m["reason"]
+            continue
+        m = VERSIONS_GEN_LINE_RE.match(line)
+        if m:
+            gen_reasons[(m["kind"], m["gen"])] = m["reason"]
+    return gen_reasons, schema_reasons
+
+
+def _truncate(text: str, width: int = 72) -> str:
+    text = text.strip()
+    if len(text) <= width:
+        return text
+    return text[: width - 1].rstrip() + "…"
+
+
 def _human_size(n: int) -> str:
     size = float(n)
     for unit in ("B", "KB", "MB", "GB", "TB"):
@@ -268,12 +309,17 @@ def _row(
     return _colorize(row, level) if level else row
 
 
+def _reason_line(reason: str, indent: int) -> str:
+    return _colorize("  " * indent + "↳ " + _truncate(reason), "reason")
+
+
 def print_status(root: Path) -> None:
     raw_root = root / "raw"
     if not raw_root.is_dir():
         print(f"no raw/ tree found under {root}")
         return
     manifest_referenced = _manifest_referenced_files(root / "pools")
+    gen_reasons, schema_reasons = _parse_versions(root / "VERSIONS.md")
     grand_total = 0
     grand_files = 0
     for kind_dir in sorted(p for p in raw_root.iterdir() if p.is_dir()):
@@ -316,6 +362,9 @@ def print_status(root: Path) -> None:
             kind_files += gen_files
 
             print(_row(gen_tag, gen_total, indent=1, level="gen", count=gen_files))
+            gen_reason = gen_reasons.get((kind, gen_tag))
+            if gen_reason:
+                print(_reason_line(gen_reason, indent=2))
             print(
                 _row(
                     "raw", raw_size, indent=2, level="bucket",
@@ -337,6 +386,9 @@ def print_status(root: Path) -> None:
                             count=s_total, referenced=s_referenced,
                         )
                     )
+                    schema_reason = schema_reasons.get((kind, gen_tag, f"schema{s}"))
+                    if schema_reason:
+                        print(_reason_line(schema_reason, indent=4))
             else:
                 print(_colorize("      (none)", "schema"))
         print(_row(f"{kind} total", kind_total, indent=1, level="gen", count=kind_files))