fix(runtime): align Web Storage accessors

[andrewtdiz]

Summary

• Make the localStorage and sessionStorage accessor getters return their canonical storage objects instead of undefined.
• Split the Storage.prototype.length getter from the global storage getter so it returns the current storage length for storage receivers and throws Illegal invocation for incompatible receivers.
• Extend the globals/storage-globals parity fixture to cover storage getter identity and Storage.prototype.length invocation behavior.

Root Cause

The runtime installed reflectable Web Storage accessor descriptors, but reused a no-op getter thunk. Descriptor getter calls therefore returned undefined, and the prototype length getter did not enforce Node's receiver behavior.

Validation

• NODE26_BIN=$(npm exec --yes --package=node@26 -- node -e 'console.log(process.execPath)') && "$NODE26_BIN" --experimental-strip-types --experimental-webstorage --localstorage-file=/tmp/perry_web_storage_parity.sqlite test-parity/node-suite/globals/storage-globals.ts
• CARGO_TARGET_DIR=/root/perry-worktrees/.build-targets/storage-globals-check RUSTC_WRAPPER= CARGO_BUILD_JOBS=1 cargo check -q -j1 -p perry-runtime
• cargo fmt --all -- --check
• git diff --check
• ./scripts/check_file_size.sh

Refs #812.

[andrewtdiz]

Closing this duplicate in favor of #4341, which covers the same Web Storage accessor slice and has the fuller internal length-slot fix plus focused parity validation.