initial: packwiz → simple-mod-sync manifest converter

Reads packwiz pack (pack.toml + index.toml + per-file .pw.toml) and
emits a simple-mod-sync sync_version=3 manifest. Drops server-only
mods, skips CurseForge metadata-mode entries with a warning, maps
content type from the parent directory of each metafile.

Optional --bundle-non-mods zips config/, options.txt etc into one
archive served as a 'packed' entry — covers the gap where simple-mod-sync
only ships zip-extractable content for non-mods.

15 tests, includes integration against upstream packwiz-example-pack.

tests/fixture-pack/shaderpacks/iris.pw.toml

@@ -0,0 +1,8 @@
+name = "Complementary Shaders"
+filename = "complementary-r5.5.zip"
+side = "client"
+
+[download]
+url = "https://example.com/complementary-r5.5.zip"
+hash-format = "sha256"
+hash = "bbbb000000000000000000000000000000000000000000000000000000000000"