test(codemode): add Test262 array coverage (#35965)

packages/codemode/test/test262-array.md

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+# Test262 Array Coverage
+
+The Array tests adapt Test262 at revision `250f204f23a9249ff204be2baec29600faae7b75`. They cover CodeMode's 35
+exposed instance methods and three static methods using actual arrays, accepted argument types, deterministic behavior,
+and CodeMode's materialized collection conventions. Each executable case names its exact upstream source path.
+`LICENSE.test262` contains the upstream BSD terms.
+
+This is coverage of CodeMode's bounded Array surface, not a claim of ECMAScript or Test262 conformance. One upstream
+file may contain both adapted and inapplicable assertions, so a cited source means only that the represented assertions
+were adapted.
+
+## Inventory
+
+The 38 relevant upstream API directories contain 2,837 files. The executable suite adapts assertions from 83 distinct
+sources.
+
+| API                             | Upstream files | Adapted sources |
+| ------------------------------- | -------------: | --------------: |
+| `Array.prototype.map`           |            216 |               3 |
+| `Array.prototype.filter`        |            242 |               3 |
+| `Array.prototype.find`          |             23 |               4 |
+| `Array.prototype.findIndex`     |             23 |               3 |
+| `Array.prototype.findLast`      |             24 |               3 |
+| `Array.prototype.findLastIndex` |             24 |               3 |
+| `Array.prototype.some`          |            219 |               2 |
+| `Array.prototype.every`         |            218 |               2 |
+| `Array.prototype.includes`      |             30 |               2 |
+| `Array.prototype.join`          |             23 |               2 |
+| `Array.prototype.reduce`        |            260 |               3 |
+| `Array.prototype.reduceRight`   |            260 |               3 |
+| `Array.prototype.flatMap`       |             24 |               2 |
+| `Array.prototype.forEach`       |            190 |               2 |
+| `Array.prototype.sort`          |             54 |               3 |
+| `Array.prototype.toSorted`      |             21 |               4 |
+| `Array.prototype.slice`         |             71 |               1 |
+| `Array.prototype.concat`        |             69 |               3 |
+| `Array.prototype.indexOf`       |            201 |               2 |
+| `Array.prototype.lastIndexOf`   |            198 |               2 |
+| `Array.prototype.at`            |             13 |               3 |
+| `Array.prototype.flat`          |             19 |               2 |
+| `Array.prototype.reverse`       |             18 |               1 |
+| `Array.prototype.toReversed`    |             17 |               2 |
+| `Array.prototype.with`          |             21 |               2 |
+| `Array.prototype.push`          |             24 |               1 |
+| `Array.prototype.pop`           |             23 |               1 |
+| `Array.prototype.shift`         |             20 |               1 |
+| `Array.prototype.unshift`       |             22 |               1 |
+| `Array.prototype.splice`        |             81 |               3 |
+| `Array.prototype.fill`          |             22 |               3 |
+| `Array.prototype.copyWithin`    |             39 |               2 |
+| `Array.prototype.keys`          |             12 |               1 |
+| `Array.prototype.values`        |             12 |               1 |
+| `Array.prototype.entries`       |             12 |               1 |
+| `Array.from`                    |             47 |               3 |
+| `Array.isArray`                 |             29 |               2 |
+| `Array.of`                      |             16 |               1 |
+
+## Exclusions
+
+Assertions are not adapted when they test behavior outside CodeMode's documented Array surface:
+
+- Function metadata, property descriptors, constructibility, prototype mutation, species constructors, or cross-realm
+  identity.
+- Generic receivers, detached methods, `.call`, `.apply`, boxed values, custom coercion objects, Symbols, BigInts,
+  proxies, accessors, frozen arrays, typed arrays, or ArrayBuffers.
+- `Array.from` mappers, custom iterables, constructor substitution, and iterator-closing behavior.
+- Native iterator identity, `.next()`, completion records, or live iterator mutation. CodeMode deliberately materializes
+  `keys`, `values`, and `entries` as arrays.
+- Sparse-array assertions that depend on literal elisions or inherited indexed properties. CodeMode's confined data
+  model does not preserve those prototype and hole semantics at every boundary.
+- Argument coercions outside the accepted schema-like surface. Numeric positions must be numbers and `join` separators
+  must be strings.
+- Exact native error brands where CodeMode exposes a safe runtime error instead.
+- Async/effectful callbacks, circular-data rejection, sandbox-value identity, diagnostics, and host-boundary behavior.
+  Those remain covered by CodeMode-specific tests.
+
+Handwritten tests remain where they specify CodeMode behavior rather than ordinary ECMAScript Array semantics.