fix(fetch): support Headers init and iteration

[andrewtdiz]

Fixes #3440
Fixes #2645

Summary

• Support new Headers(init) for records, arrays/sequences, Maps/custom sync iterables, and existing Headers clones.
• Coerce header init names and values through JS string conversion, and throw TypeError for null init or malformed constructor pairs.
• Make first-class Headers method reads callable and stable enough for collection usage, with Symbol.iterator/@@iterator aliasing entries so spread, Array.from, and for...of work.

Why batched

Both issues target the same Headers collection semantics surface. Constructor materialization, iterator lookup, and runtime method dispatch share the same code paths, so fixing them together avoids splitting one behavior change across multiple PRs.

Tests

• cargo fmt --all
• cargo check -p perry-runtime -p perry-stdlib -p perry-codegen -p perry
• PERRY_NO_AUTO_OPTIMIZE=1 ./run_parity_tests.sh --suite node-suite --module fetch --filter headers
• PERRY_NO_AUTO_OPTIMIZE=1 CARGO_TARGET_DIR=/tmp/perry-headers-target cargo build --release -p perry -p perry-runtime -p perry-stdlib
• Manual isolated parity compare with /tmp/perry-headers-target/release/perry:
  • test-parity/node-suite/fetch/headers/append-combines.ts
  • test-parity/node-suite/fetch/headers/constructor-init.ts
  • test-parity/node-suite/fetch/headers/get-set-cookie.ts
  • test-parity/node-suite/fetch/headers/iterable-symbol.ts
• cargo fmt --all -- --check
• git diff --check
• ./scripts/check_file_size.sh

Non-goals

• This does not broaden unrelated fetch/body behavior.
• This does not address other Headers edge cases beyond constructor init, cloning, iterator lookup, and first-class method reads covered by the linked issues and added parity cases.

[andrewtdiz]

Updated this branch for the current mainline.

• Merged origin/main through 04d9936e3.
• Rebuilt the release CLI and regenerated the API manifest outputs; docs were already current after the merge.
• Local checks passed: cargo fmt --all -- --check, git diff --check, ./scripts/check_file_size.sh, CARGO_TARGET_DIR=/root/perry/target cargo test -p perry-api-manifest sys_alias_mirrors_util_manifest, RUST_TEST_THREADS=1 CARGO_TARGET_DIR=/root/perry/target cargo test -p perry-stdlib headers, and CARGO_TARGET_DIR=/root/perry/target cargo test -p perry-hir stable_hash::tests::expr_variant_stable_hash_tags_are_unique.