fix(deps): move futures-util to 0.3.34, dropping pin-utils (#273)

RUSTSEC-2026-0441 marks `pin-utils` unmaintained; its upstream
repository was archived on 2026-10-01. On `main`, `cargo audit` reports
it through `futures-util` 0.3.31.

`futures-util` 0.3.32 and later no longer depend on `pin-utils`. This
runs `cargo update -p futures-util`, which:

- moves `futures-core`, `futures-macro`, `futures-task` and
`futures-util` from 0.3.31 to 0.3.34;
- removes `pin-utils` from the lockfile.

Only `Cargo.lock` changes.

## Path in

`futures-util` reaches the build two ways:

- through the GUI's accessibility chain: `eframe` → `egui-winit` →
`accesskit_winit` → `accesskit_unix`;
- through `js-sys`.

## Verification

- `cargo audit` exits 0. Its warnings drop from 5 to 4. The remaining
four are the upstream-blocked unmaintained advisories, W1 and W2 in
`docs/UPSTREAM-DEPENDENCY-WATCH.adoc` in hyperpolymath/standards.
- `cargo clippy --all-features --all-targets -- -D warnings` passes.
- `cargo test --all-features` passes, including the 531-test main suite.
- `futures-util` 0.3.34 declares `rust-version` 1.71, below this crate's
1.90.

---
_Generated by [Claude
Code](https://claude.ai/code/session_01Jma5ixtpeZyJRmfoSsC98u)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath