fix(changelog): stop prepending the preamble on every run

write_md() put the H1 + description inside the generated body and also kept
old[:m.start()], so each regeneration added another copy: 44 runs left 44 H1s and
pushed the first release to line 179. Only the region between the markers is
generated now, an existing file with N copies is collapsed to one, and an assert
refuses to write a file containing two generated regions (proven RED by injecting
a duplicate). Regenerated: 83 entries, 185 commits, first heading v4.6.146 (was
v4.6.135 - the file had not been committed since 2026-11-02).

1 files changed, 34 insertions, 12 deletions

diff --git a/scripts/build-changelog.py b/scripts/build-changelog.py
index 1b00a5e..8973761 100644
--- a/scripts/build-changelog.py
+++ b/scripts/build-changelog.py
@@ -275,21 +275,31 @@ def write_page(page):
     return len(new)
 
 
+PREAMBLE = ('# newOS4 changelog\n\n'
+            'Generated from git history by `scripts/build-changelog.py` — do not edit by hand '
+            'between the markers.\n\n')
+# The preamble is emitted EXACTLY once. Before 2026-11-05 it was part of the generated body *and*
+# old[:m.start()] was preserved on top of it, so each run prepended another copy: 44 runs left 44
+# copies of the H1 at the top of the file (and pushed the real first release from line 6 to 179).
+PREAMBLE_RE = re.compile(re.escape('# newOS4 changelog') +
+                         r'\n\nGenerated from git history by `scripts/build-changelog\.py` — '
+                         r'do not edit by hand between the markers\.\n+')
+
+
 def write_md(page, rows):
-    """App-side Keep-a-Changelog history; generated region is delimited."""
-    out = ['# newOS4 changelog', '',
-           'Generated from git history by `scripts/build-changelog.py` — do not edit by hand '
-           'between the markers.', '', '<!-- generated:start -->', '']
+    """App-side Keep-a-Changelog history; only the region between the markers is generated."""
+    region = ['<!-- generated:start -->', '']
     for entry in page['entries']:
-        out.append('## %s — %s' % (entry['version'], entry['date']))
-        out.append('')
+        region.append('## %s — %s' % (entry['version'], entry['date']))
+        region.append('')
         if entry['note']:
-            out.extend(['_' + re.sub(r'<[^>]+>', '', entry['note']) + '_', ''])
+            region.extend(['_' + re.sub(r'<[^>]+>', '', entry['note']) + '_', ''])
         for item in entry['items']:
-            out.append('- ' + re.sub(r'<[^>]+>', '`', item).replace('``', '`').replace('``', '`'))
-        out.append('')
-    out.extend(['<!-- generated:end -->', ''])
-    body = '\n'.join(out)
+            region.append('- ' + re.sub(r'<[^>]+>', '`', item).replace('``', '`').replace('``', '`'))
+        region.append('')
+    region.extend(['<!-- generated:end -->', ''])
+    region_text = '\n'.join(region)
+
     if os.path.exists(MD):
         with open(MD, encoding='utf-8') as fh:
             old = fh.read()
@@ -299,7 +309,19 @@ def write_md(page, rows):
             # hand-edit added, and silently dropping it is worse than failing loudly.
             raise SystemExit('CHANGELOG.md exists but has no generated:start/end markers - '
                              'refusing to overwrite it')
-        body = old[:m.start()] + body + old[m.end():]
+        head, tail = old[:m.start()], old[m.end():]
+        copies = len(PREAMBLE_RE.findall(head))
+        if copies != 1:
+            rest = PREAMBLE_RE.sub('', head).strip('\n')
+            head = PREAMBLE + (rest + '\n\n' if rest else '')
+            print('changelog: collapsed %d preamble copies in CHANGELOG.md' % copies)
+        body = head + region_text + tail
+    else:
+        body = PREAMBLE + region_text
+    # Guard against the 2026-11-05 regression class: the generated region may appear exactly once.
+    # The old code concatenated the preamble into `body` and kept old[:m.start()] as well, so every
+    # run appended another copy — 44 runs, 44 H1s, and the first release pushed to line 179.
+    assert body.count('<!-- generated:start -->') == 1, 'CHANGELOG.md would contain a duplicated region'
     write_atomic(MD, body)
     return len(body)