⚡ perf: memoize per-class plans; add depth guard and per-index each reporting

Profiling the validator showed roughly half of all validation time re-deriving
answers that cannot change — collectConstraints 22%, getOwnMetadata 12%,
getMetadataChain 9%, getProperties 4%, getMetadata 3%, plus 8% GC from the
allocation churn. The constraint predicates themselves were under 1%.

Decorator metadata is fixed once classes are declared, so the derived structures
are now memoized per prototype: the validation plan, the serialization plan, the
deserialization plan, and serializer/deserializer instances, which were being
constructed fresh for every property of every object. MetadataStorage carries a
version counter that invalidates the caches when metadata is written, so
registerDecorator after first use still works — covered by a test.

Measured against JSON.parse + JSON.stringify as a fixed reference:

  validate    (50 orders)   221.6 us -> 49.6 us   4.5x
  validate    (10 orders)    47.8 us -> 12.9 us   3.7x
  toInstance  (50 orders)   255.1 us -> 74.0 us   3.4x
  toInstance  (10 orders)    64.6 us -> 19.0 us   3.4x
  toPlain     (50 orders)   294.4 us -> 95.3 us   3.1x

Reliability, in the same pass:

- maxDepth option (default 64) on every mapping function, on validate(), and on
  configure(). All three engines recurse, so a payload nested thousands of levels
  deep could exhaust the call stack. Cycles were already handled; legitimate deep
  nesting was not bounded.
- each: true failures now name the element that failed ("failed at index 3"). A
  bad entry in a 200-item array previously produced a message that could not
  locate it. A message function now receives the failing element as args.value
  rather than the whole array; caller-supplied strings stay verbatim.

150 tests (up from 136), all green on the existing suite unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SAcqrz3FcadkYr3xG32CjK

README.md

@@ -203,6 +203,7 @@ defaults for the whole application. Per-call options win.
 | `validate` | `boolean` | `true` | Validate the result; throw `JsonValidationError` on failure. |
 | `namingStrategy` | strategy name or function | `identity` | JSON naming convention for properties without `@JsonProperty`. |
 | `unknownKeys` | `allow` \| `strip` \| `error` | `allow` | What to do with incoming keys matching no declared property. |
+| `maxDepth` | `number` | `64` | Nesting depth before a `JsonMappingError` is raised, bounding hostile payloads. |
 
 ```typescript
 // lenient parse: build the instance, inspect the damage yourself
@@ -293,7 +294,7 @@ Write your own with `registerDecorator({ name, target, propertyName, validator }
 - `toInstance(clazz, plain, options?)`: Transforms a plain object to a validated class instance (`Promise<T>`).
 - `toInstanceArray(clazz, plain, options?)`: Same, for an array (`Promise<T[]>`).
 - `fromRequest(clazz, request, options?)`: Extracts JSON from a Fetch `Request` (`Promise<T>`).
-- `validate(obj)`: Full validation, returning `Promise<ValidationError[]>`.
+- `validate(obj, options?)`: Full validation, returning `Promise<ValidationError[]>`.
 - `validateOrReject(obj)`: As above, but throws `JsonValidationError`.
 - `configure(options)` / `getConfig()` / `resetConfig()`: Library-wide defaults.
 
@@ -350,6 +351,26 @@ app.post('/user', async (req, res) => {
 });
 ```
 
+## Performance
+
+Decorator metadata is fixed once your classes are declared, so cereale resolves each class's
+validation, serialization and deserialization plans once and memoizes them per prototype.
+A version counter invalidates the caches if metadata is registered late, so `registerDecorator`
+after first use still behaves correctly.
+
+Indicative throughput for a customer record with a nested address and 10 orders, measured
+against `JSON.parse` + `JSON.stringify` (5.9 us) on the same machine:
+
+| Operation | Time |
+| --- | --- |
+| `toInstance` (deserialize + validate) | ~19 us |
+| `toInstance` with `{ validate: false }` | ~6 us |
+| `validate` on an existing instance | ~13 us |
+| `toPlain` (validate + serialize) | ~23 us |
+
+If you validate at the edge and map internally afterwards, `{ validate: false }` skips the
+dominant cost.
+
 ## Notes and Limitations
 
 - **Circular references** are rejected during serialization with a `JsonMappingError`. Break