fix(fetch): implement Request.text()/.json()/.arrayBuffer() (#1688)

[proggeramlug]

Summary

Implements Request.text() / .json() / .arrayBuffer() under Perry-native compile. Closes #1688.

Before this change, new Request(url, { body }).text() / .json() / .arrayBuffer() returned 0 instead of the request body. req.method / req.url / req.headers already worked (#1649); only the body-consuming methods were unimplemented — the typed module=="Request" codegen arm in lower_call/options/fetch.rs handled the property getters but had no body-method dispatch, so req.text() fell through to a generic path that yielded 0.

Changes

• Runtime (perry-stdlib): add js_request_text / js_request_json / js_request_array_buffer, mirroring the existing Response js_fetch_response_text / js_fetch_response_json / js_response_array_buffer path. Body is in-memory, so the promise resolves synchronously (the LLVM await loop doesn't drain the deferred pump). A bodiless request reads as "".
• Runtime (perry-ext-fetch): same three FFIs for the well-known-flip path, matching that crate's conventions, plus a data-path unit test.
• Codegen: route text / json / arrayBuffer in the Request arm to the new FFIs (Promise pointer NaN-boxed as POINTER_TAG); declare them in runtime_decls.
• Test: extend test_gap_fetch_response.ts with the three body methods.

Validation

Byte-for-byte identical to node --experimental-strip-types on the full test_gap_fetch_response.ts gap test (including the new request text / request json data / request arrayBuffer byteLength lines). cargo test -p perry-ext-fetch green (7 tests). cargo fmt --all -- --check clean.

Both code paths were exercised: the well-known flip routed fetch → perry-ext-fetch and linked cleanly, confirming the ext-fetch additions are also reached.

Note (out of scope)

Using new Request(...) inline (e.g. await new Request(...).text()) doesn't set the uses_fetch feature flag, so the auto-optimize build omits the fetch module and the link fails — a pre-existing feature-detection limitation that affects all inline-Request method calls (and js_request_new itself), not just the new ones. Assigning to a variable first (const r = new Request(...), as in the #1688 repro) works. Tracked separately in #1691.