diff-based mod removal via --previous-manifest

Adds removal-detection: when --previous-manifest <path> is given,
the converter diffs the previous publish against current packwiz state
and emits modify[].type=remove entries for mods/resourcepacks/etc that
disappeared, using simple-mod-sync's on-disk naming convention as the
regex pattern.

Reverse-engineered from upstream source:
- simple-mod-sync writes <sanitized_name>-<sanitized_version>.<ext>
- StringUtils.sanitize strips [^a-zA-Z0-9.\-_]
- GetOlderVersion() finds files starting with <name>- and auto-deletes
  on version bumps. So version upgrades need no converter handling;
  only full removals do.

8 new tests including end-to-end CLI verification with a synthetic
previous manifest. 23/23 pass.

tests/test_packwiz_to_sms.py

@@ -168,6 +168,112 @@ def test_cli_missing_pack_toml(tmp_path: Path) -> None:
     assert "pack.toml" in proc.stderr
 
 
+def test_diff_removals_empty_when_no_changes() -> None:
+    """No removals when current sync covers everything in previous."""
+    prev = [{"name": "Sodium", "type": "mod"}, {"name": "Fabric API", "type": "mod"}]
+    cur = [{"name": "Sodium", "type": "mod"}, {"name": "Fabric API", "type": "mod"}]
+    assert p2s.diff_removals(prev, cur) == []
+
+
+def test_diff_removals_emits_remove_for_dropped_mod() -> None:
+    """Mod present previously but not currently → modify[].type=remove entry."""
+    prev = [{"name": "Sodium", "type": "mod"}, {"name": "Iris", "type": "mod"}]
+    cur = [{"name": "Sodium", "type": "mod"}]
+    rm = p2s.diff_removals(prev, cur)
+    assert len(rm) == 1
+    assert rm[0]["type"] == "remove"
+    assert rm[0]["path"] == "."
+    # pattern targets simple-mod-sync's on-disk naming
+    assert rm[0]["pattern"] == r"^mods/Iris-.*\.jar$"
+
+
+def test_diff_removals_sanitizes_name() -> None:
+    """Name with spaces and symbols sanitized to match simple-mod-sync's filename."""
+    prev = [{"name": "Fabric API (Old)", "type": "mod"}]
+    cur: list[dict] = []
+    rm = p2s.diff_removals(prev, cur)
+    # sanitize() strips space, '(', ')'
+    assert rm[0]["pattern"] == r"^mods/FabricAPIOld-.*\.jar$"
+
+
+def test_diff_removals_per_type_paths_and_extensions() -> None:
+    """Each content type targets the right directory + extension."""
+    prev = [
+        {"name": "Branding", "type": "resourcepack"},
+        {"name": "Iris", "type": "shader"},
+        {"name": "Worldgen", "type": "datapack"},
+    ]
+    rm = p2s.diff_removals(prev, [])
+    patterns = {r["pattern"] for r in rm}
+    assert patterns == {
+        r"^resourcepacks/Branding-.*\.zip$",
+        r"^shaderpacks/Iris-.*\.zip$",
+        r"^datapacks/Worldgen-.*\.zip$",
+    }
+
+
+def test_convert_with_previous_manifest_adds_modify_section() -> None:
+    """When --previous-manifest is supplied to convert(), modify[] appears
+    in output for any name that disappeared."""
+    previous = {
+        "sync_version": 3,
+        "sync": [
+            {"name": "Sodium", "type": "mod", "url": "x", "version": "1"},
+            {"name": "Iris", "type": "mod", "url": "y", "version": "1"},
+            {"name": "Some Removed Mod", "type": "mod", "url": "z", "version": "1"},
+        ],
+    }
+    r = p2s.convert(FIXTURE, previous_manifest=previous)
+    assert "modify" in r.manifest
+    patterns = {e["pattern"] for e in r.manifest["modify"]}
+    # Fixture has Sodium + Fabric API (both kept under "mod"), so Iris + Some Removed Mod
+    # should both be removed. (Iris in fixture is type=shader under "Complementary Shaders",
+    # so the mod-keyed Iris from previous is genuinely gone.)
+    assert r"^mods/Iris-.*\.jar$" in patterns
+    assert r"^mods/SomeRemovedMod-.*\.jar$" in patterns
+
+
+def test_convert_without_previous_manifest_omits_modify() -> None:
+    r = p2s.convert(FIXTURE)
+    assert "modify" not in r.manifest
+    assert r.removed_entries == []
+
+
+def test_cli_previous_manifest_flag(tmp_path: Path) -> None:
+    """End-to-end CLI with --previous-manifest."""
+    prev = tmp_path / "old.json"
+    prev.write_text(json.dumps({
+        "sync_version": 3,
+        "sync": [
+            {"name": "Sodium", "type": "mod", "url": "x", "version": "1"},
+            {"name": "GoneAway", "type": "mod", "url": "y", "version": "1"},
+        ],
+    }))
+    proc = subprocess.run(
+        [sys.executable, str(SCRIPT), str(FIXTURE), "--quiet",
+         "--previous-manifest", str(prev)],
+        capture_output=True, text=True, check=True,
+    )
+    payload = json.loads(proc.stdout)
+    assert "modify" in payload
+    patterns = {e["pattern"] for e in payload["modify"]}
+    assert r"^mods/GoneAway-.*\.jar$" in patterns
+
+
+def test_cli_previous_manifest_missing_file_warns(tmp_path: Path) -> None:
+    """Non-existent previous-manifest is a warning, not an error — first run case."""
+    nonexistent = tmp_path / "never-written.json"
+    proc = subprocess.run(
+        [sys.executable, str(SCRIPT), str(FIXTURE),
+         "--previous-manifest", str(nonexistent)],
+        capture_output=True, text=True, check=True,
+    )
+    # Should succeed, emit manifest with no modify section, and warn on stderr.
+    payload = json.loads(proc.stdout)
+    assert "modify" not in payload
+    assert "does not exist" in proc.stderr
+
+
 def test_convert_against_real_packwiz_example_pack(tmp_path: Path) -> None:
     """Integration: clone the upstream packwiz-example-pack and convert it.