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.

README.md

@@ -33,8 +33,26 @@ python3 packwiz_to_sms.py /path/to/packwiz/pack \
     -o manifest.json \
     --bundle-non-mods overrides.zip \
     --bundle-url https://packs.example.com/overrides.zip
+
+# Generate removal entries for mods dropped since the previous publish
+python3 packwiz_to_sms.py /path/to/packwiz/pack \
+    -o manifest.json \
+    --previous-manifest /path/to/last-published-manifest.json
 ```
 
+## Mod removal
+
+Two cases, handled differently:
+
+| What changed | Who handles cleanup |
+|---|---|
+| **Mod version bump** (Sodium 0.5 → 0.6) | simple-mod-sync itself — it writes downloaded files as `<sanitized_name>-<sanitized_version>.<ext>` and on update looks for any prior file starting with `<name>-` and deletes it. No converter intervention needed. |
+| **Mod removed entirely** (no longer in pack) | Converter emits an explicit `modify[].type=remove` entry with a regex matching simple-mod-sync's on-disk naming. Triggered by `--previous-manifest <path>` flag. |
+
+Without `--previous-manifest`, removed mods stay on player disk forever. In CI, keep the last-published manifest and feed it in as the previous one on every run.
+
+The regex follows simple-mod-sync's `StringUtils.sanitize()` rules: `[^a-zA-Z0-9.\-_]` characters are stripped. So `"Fabric API (Old)"` → pattern `^mods/FabricAPIOld-.*\.jar$`.
+
 ## What gets emitted
 
 | Packwiz path | Becomes simple-mod-sync `type` |
@@ -51,7 +69,6 @@ CurseForge mods that use `mode = "metadata:curseforge"` (no direct URL) are skip
 ## What's not handled
 
 - **Optional mods** — simple-mod-sync has no per-client toggle UI. All non-server mods are emitted unconditionally. Ship two manifests (with/without optional) if you need this.
-- **Mod removal** — simple-mod-sync's `modify`/`remove` is not auto-populated. Convert by hand if you're dropping mods.
 - **Rename / regex transforms** — packwiz has no equivalent concept, so we don't generate `modify.rename` entries.
 
 ## How it works