chore: pomelo workspace checks unfucking

[hawkw]

This branch makes two changes:

1. Turns out I had forgotten to add pomelo to the Just commands that run a
   check for all crates, like just check, just clippy, and just docs. So,
   it was never actually being checked, on CI or in VS Code (MY BAD LMAO).
   Therefore, this branch adds it to those recipes so it actually gets checked.

2. Once we actually include pomelo in just check, it turns out that the
   build is fucked, because Cargo doesn't realize what target to build pomelo
   for. Thus, I've also added a missing forced-target="wasm32-unknown-unknown
   to pomelo's Cargo.toml, which should unfuck it.

3. Also I fixed an "unnecessary mut" warning, because now that we're actually
   linting pomelo on CI, it turns out that it, uh, has lints. Whoops.

[hawkw]

well, now CI fails for an actually legitimate reason, because pomelo has one warning in it: https://github.com/tosc-rs/mnemos/actions/runs/6018848174/job/16327759771?pr=263#step:8:1186