news-digest: separated SOURCE / REDDIT THREAD action chips + archive page

Two requested polish items:

1. Reddit items now show TWO distinct action chips in the footer:

     [↗ SOURCE huggingface.co]  [⌥ REDDIT THREAD]      3h · u/foo

   Previously the external link was a tiny dot-separated text link
   that visually competed with the post metadata. Now: explicit
   labeled buttons, distinct colors (cyan for source, blue for
   thread), with hover states that match the Australis accent
   palette. Non-Reddit items get just the SOURCE chip.

2. Archive page at /archive.html — lists every edition-*.html on
   disk, newest-first, sorted PM-before-AM within a day. Each row
   is the date in editorial serif + an AM/PM chip color-coded
   (yellow morning / cyan evening) + a hover-affordance arrow.
   Header link from the main edition reads "ARCHIVE →".

   Generation: digest.py walks OUTPUT_DIR for the edition-*.html
   filename pattern on every run, sorts, renders archive.html.j2,
   writes atomically. Cheap (~1ms even with hundreds of editions).
   No retention cap — twice-daily for a year is ~700 small files,
   well under any reasonable concern.

CSS additions: .action chip styles (with per-action color variants),
.archive-link in masthead, full .archive-row + .archive-edition
treatment.

stacks/news-digest/digest.py

@@ -353,6 +353,54 @@ def write_output(html: str, generated_at: datetime) -> None:
     index_tmp.write_text(html, encoding="utf-8")
     index_tmp.rename(index)
     log(f"wrote {index} (and archive {archive.name})")
+    write_archive_index(generated_at)
+
+ARCHIVE_FNAME_RE = re.compile(r"^edition-(\d{4}-\d{2}-\d{2})-(am|pm)\.html$")
+
+def write_archive_index(generated_at: datetime) -> None:
+    """Render /output/archive.html — list every edition-*.html in
+    OUTPUT_DIR, newest-first. Cheap (re-runs every digest fire);
+    template loads from the same TEMPLATE_DIR."""
+    editions = []
+    for p in OUTPUT_DIR.glob("edition-*.html"):
+        m = ARCHIVE_FNAME_RE.match(p.name)
+        if not m:
+            continue
+        date_str, ed = m.group(1), m.group(2)
+        try:
+            date = datetime.strptime(date_str, "%Y-%m-%d").date()
+        except ValueError:
+            continue
+        editions.append({
+            "filename": p.name,
+            "date": date,
+            "edition": "morning" if ed == "am" else "evening",
+            "edition_short": ed.upper(),
+            "date_long": date.strftime("%A %B %-d, %Y"),
+            # Sort key: date descending, then PM before AM (within a day,
+            # PM is the most recent edition).
+            "_sort": (date, 1 if ed == "pm" else 0),
+        })
+    editions.sort(key=lambda e: e["_sort"], reverse=True)
+
+    env = Environment(
+        loader=FileSystemLoader(str(TEMPLATE_DIR)),
+        autoescape=select_autoescape(["html"]),
+        trim_blocks=True,
+        lstrip_blocks=True,
+    )
+    tpl = env.get_template("archive.html.j2")
+    html = tpl.render(
+        editions=editions,
+        generated_at=generated_at,
+        total=len(editions),
+    )
+
+    out = OUTPUT_DIR / "archive.html"
+    out_tmp = out.with_suffix(".html.tmp")
+    out_tmp.write_text(html, encoding="utf-8")
+    out_tmp.rename(out)
+    log(f"wrote {out} ({len(editions)} editions indexed)")
 
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