compat: auto-generated TypeScript feature matrix

[TheHypnoo]

For each TS feature (closures, async, generators, classes, mixins, decorators, generics, optional chaining, type-only imports, …) emit N micro-tests with pass/fail. Regenerate test-features/feature_matrix.md in CI.

Part of #793.

[proggeramlug]

Initial feature-matrix scaffolding landed (this branch). The matrix surfaced 4 real Perry gaps on its first run — filed as standalone bugs so they're trackable independently:

• optional call obj.fn?.(...) throws when fn is undefined (should short-circuit to undefined) #830 — optional call `obj.fn?.(...)` throws instead of short-circuiting
• segfault when import { type X } and import { X as Y } come from the same module #831 — segfault when type-only + value imports share a name from the same module
• decorator that returns the target class is rejected; Node accepts it #832 — decorator returning the target class is rejected (Node accepts it)
• String.prototype.replace(/.../g, fn) — replacer-fn return value is dropped (replaces with empty string) #833 — `String.replace(re, fn)` drops the replacer's return value

Scaffolding committed:

• `test-features/feature_matrix.toml` — category → probe-file mapping
• `scripts/gen_feature_matrix.py` — runs all probes against Perry + Node, writes `test-features/feature_matrix.md`
• New probes: `test_feat_classes_basic.ts`, `test_feat_mixins.ts`, `test_feat_decorators.ts`, `test_feat_optional_chaining.ts`, `test_feat_type_only_imports.ts`

First-run summary (18 categories, 25 probes):
```
closures PASS (1/1)
async PASS (1/1)
generators PASS (1/1)
classes PASS (2/2)
mixins PASS (1/1) ← #740 fix permanently pinned
decorators 1 fail (0/1) → #832
optional_chaining 1 fail (0/1) → #830
type_only_imports 1 fail (0/1) → #831
regexp 1 fail (0/1) → #833
bigint PASS (1/1)
symbols PASS (1/1)
map_set PASS (1/1)
proxy_reflect PASS (1/1)
typed_arrays PASS (1/1)
json PASS (1/1)
dynamic_import PASS (8/8)
weakref PASS (1/1)
error_handling PASS (1/1)
```

Next: wire `scripts/gen_feature_matrix.py --check` into CI as an advisory job and commit `feature_matrix.md` so PRs can show diff-as-review.

[andrewtdiz]

I am taking a focused current-main cut for #801. Scope: recreate the feature-matrix scaffolding, commit the generated markdown, and wire an advisory CI check that verifies the matrix is up to date. I will keep this to tracking/reporting rather than fixing feature gaps surfaced by the matrix.

[andrewtdiz]

Opened draft PR #4396 for the TypeScript feature matrix radar.

Scope:

• test-features probe config, probes, README, and committed markdown baseline
• scripts/gen_feature_matrix.py for Node-vs-Perry markdown/JSON generation and --check drift mode
• advisory scheduled/manual/PR workflow for matrix drift checks

Current baseline: 22 probes across 14 categories; 19 pass / 3 recorded diffs.

Validation:

• Node v26 feature matrix check with JSON/generated markdown artifacts
• release build: cargo build --release -p perry -p perry-runtime -p perry-stdlib
• python3 -m py_compile scripts/gen_feature_matrix.py
• git diff --check
• ./scripts/check_file_size.sh

[andrewtdiz]

Updated draft PR #4396 with follow-up commit f5d324f.

The feature-matrix workflow now builds the Perry compiler/runtime targets used by the probes: cargo build --release -p perry-runtime -p perry.

Latest validation on current main:

• release build with that workflow target list
• Node v26 feature matrix check with JSON/generated markdown artifacts
• python3 -m py_compile scripts/gen_feature_matrix.py
• workflow YAML parse
• git diff --check origin/main..HEAD and git diff --check
• ./scripts/check_file_size.sh