Add numcodecs namespace and built-in shuffle/delta codecs

V2 filters and compressors are now looked up under a `numcodecs.` prefix
in the codec registry, matching zarr-python's naming convention. This
separates the v2-specific codec namespace from v3 codecs and makes it
clear where these codecs come from.

Existing codecs that serve both v2 and v3 (blosc, zstd, gzip, zlib, etc.)
are registered under both names so nothing breaks. Adds `numcodecs.shuffle`
and `numcodecs.delta` as built-in codecs since these are commonly requested
v2 filters (#311, #315) and are trivial pure-JS implementations.

Author: manzt

.changeset/numcodecs-namespace.md

@@ -0,0 +1,27 @@
+---
+"zarrita": minor
+---
+
+Add `numcodecs.` namespace for v2 codec registry and built-in shuffle/delta codecs
+
+V2 filters and compressors are now registered under a `numcodecs.` prefix
+(e.g., `numcodecs.blosc`, `numcodecs.zlib`) matching zarr-python's convention.
+This separates v2-specific codecs from the v3 codec namespace.
+
+Adds built-in `numcodecs.shuffle` and `numcodecs.delta` codecs for reading v2
+data that uses these common numcodecs filters. Both are pure JS with no WASM
+dependencies.
+
+Custom v2 codecs can be registered under the same namespace:
+
+```ts
+zarr.registry.set("numcodecs.my-filter", async () => ({
+  fromConfig(config) {
+    return {
+      kind: "bytes_to_bytes",
+      encode(data) { /* ... */ },
+      decode(data) { /* ... */ },
+    };
+  },
+}));
+```

fixtures/v2/data.zarr/.zmetadata

@@ -166,6 +166,56 @@
             ],
             "zarr_format": 2
         },
+        "1d.contiguous.delta.i2/.zarray": {
+            "chunks": [
+                4
+            ],
+            "compressor": {
+                "id": "zlib",
+                "level": 1
+            },
+            "dtype": "<i2",
+            "fill_value": 0,
+            "filters": [
+                {
+                    "astype": "<i2",
+                    "dtype": "<i2",
+                    "id": "delta"
+                }
+            ],
+            "order": "C",
+            "shape": [
+                4
+            ],
+            "zarr_format": 2
+        },
+        "1d.contiguous.delta.shuffle.i2/.zarray": {
+            "chunks": [
+                4
+            ],
+            "compressor": {
+                "id": "zlib",
+                "level": 1
+            },
+            "dtype": "<i2",
+            "fill_value": 0,
+            "filters": [
+                {
+                    "astype": "<i2",
+                    "dtype": "<i2",
+                    "id": "delta"
+                },
+                {
+                    "elementsize": 2,
+                    "id": "shuffle"
+                }
+            ],
+            "order": "C",
+            "shape": [
+                4
+            ],
+            "zarr_format": 2
+        },
         "1d.contiguous.f4.be/.zarray": {
             "chunks": [
                 4
@@ -277,6 +327,28 @@
             ],
             "zarr_format": 2
         },
+        "1d.contiguous.shuffle.i2/.zarray": {
+            "chunks": [
+                4
+            ],
+            "compressor": {
+                "id": "zlib",
+                "level": 1
+            },
+            "dtype": "<i2",
+            "fill_value": 0,
+            "filters": [
+                {
+                    "elementsize": 2,
+                    "id": "shuffle"
+                }
+            ],
+            "order": "C",
+            "shape": [
+                4
+            ],
+            "zarr_format": 2
+        },
         "1d.contiguous.u1/.zarray": {
             "chunks": [
                 4

fixtures/v2/data.zarr/1d.contiguous.delta.i2/.zarray

@@ -0,0 +1,23 @@
+{
+    "chunks": [
+        4
+    ],
+    "compressor": {
+        "id": "zlib",
+        "level": 1
+    },
+    "dtype": "<i2",
+    "fill_value": 0,
+    "filters": [
+        {
+            "astype": "<i2",
+            "dtype": "<i2",
+            "id": "delta"
+        }
+    ],
+    "order": "C",
+    "shape": [
+        4
+    ],
+    "zarr_format": 2
+}

fixtures/v2/data.zarr/1d.contiguous.delta.i2/0

@@ -0,0 +1,27 @@
+{
+    "chunks": [
+        4
+    ],
+    "compressor": {
+        "id": "zlib",
+        "level": 1
+    },
+    "dtype": "<i2",
+    "fill_value": 0,
+    "filters": [
+        {
+            "astype": "<i2",
+            "dtype": "<i2",
+            "id": "delta"
+        },
+        {
+            "elementsize": 2,
+            "id": "shuffle"
+        }
+    ],
+    "order": "C",
+    "shape": [
+        4
+    ],
+    "zarr_format": 2
+}

fixtures/v2/data.zarr/1d.contiguous.delta.shuffle.i2/.zarray

@@ -0,0 +1,22 @@
+{
+    "chunks": [
+        4
+    ],
+    "compressor": {
+        "id": "zlib",
+        "level": 1
+    },
+    "dtype": "<i2",
+    "fill_value": 0,
+    "filters": [
+        {
+            "elementsize": 2,
+            "id": "shuffle"
+        }
+    ],
+    "order": "C",
+    "shape": [
+        4
+    ],
+    "zarr_format": 2
+}