feat(pattern): canonical dict/JSON serialization for Pattern (#277)

Adds canonical serialization to `Pattern` — round-trip-safe dict and
JSON, with a schema-version header and a public `kind`-based AST
vocabulary that shields the wire format from internal class-name churn.

## Usage

```python
from edify import Pattern

p = Pattern().start_of_input().named_capture("year").exactly(4).digit().end().ignore_case()

blob = p.to_json()
# {"edify":0,"flags":{"ignore_case":true},"pattern":{"children":[...],"kind":"root"}}

restored = Pattern.from_json(blob)
assert restored == p
```

## Schema shape

```json
{
  "edify": 0,
  "pattern": {
    "kind": "root",
    "children": [
      { "kind": "start" },
      {
        "kind": "named-capture",
        "name": "year",
        "children": [
          { "kind": "exactly", "times": 4, "child": { "kind": "digit" } }
        ]
      }
    ]
  },
  "flags": { "ignore_case": true }
}
```

Every element carries a public `kind` string (registered in
`edify.serialize.kinds`). Internal class names (`ExactlyElement`, etc.)
never appear in the payload, so later privatisation and refactors don't
leak into serialized output. The `flags` key is omitted when no flags
are set.

## Schema version

`SCHEMA_VERSION = 0` — **experimental**. The concrete AST shape may
change without a deprecation cycle until it is promoted to `1` in a
later release. Consumers should not commit to the on-wire shape yet.

`from_dict` / `from_json` reject any payload whose `edify` key does not
match this build's `SCHEMA_VERSION`, and reject unknown `kind` values
with `UnknownElementKindError` so payloads from a newer build fail
loudly on an older loader.

## Forward-compatibility

Unknown fields on individual element dicts are silently ignored, so a
newer edify that adds `{"kind": "digit", "future_marker": true}` will
still load on this build — the extra field just drops.

Closes #193
Closes #194
Closes #195
Closes #196
Closes #197

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