ci: cover decode-only configuration

[anisayakmitra-in]

Fixes #161. Extends the existing no-default-features job to test the decode-only build, and extends the existing clippy job to lint both no-default configurations. Keeping the checks in their current jobs preserves the required-green wiring and avoids extra runners. Verification: cargo test -p termlens --lib --no-default-features --features decode; cargo clippy -p termlens --lib --no-default-features -- -D warnings; cargo clippy -p termlens --lib --no-default-features --features decode -- -D warnings; git diff --check; DCO and attribution checks. I did not run the full all-targets command locally because this Windows host cannot compile the repository's Unix-only integration test target; the PR CI runs it on Linux.

[vyncint]

Thank you — this is exactly the shape the issue was hoping for, and it closes a
gap that had bitten three times.

You took the option the issue suspected was better and I agree: folding the
configurations into the existing clippy and no-default-features jobs rather
than adding runners. That also means required-green needs no change, which is
the trap #143 left and the one I was most curious whether this would step in.
It didn't.

I checked that the new commands actually ran rather than trusting the green
tick, since a CI change that silently doesn't execute is the failure mode here:

clippy job:
  cargo clippy --workspace --all-targets --all-features -- -D warnings
  cargo clippy --workspace --all-targets --no-default-features -- -D warnings
  cargo clippy --workspace --all-targets --no-default-features --features decode -- -D warnings

no-default-features job:
  cargo test --workspace --no-default-features
  cargo test --workspace --no-default-features --features decode

All five ran, all passed. All four feature combinations are now built, and
clippy covers three of them instead of one.

Also appreciated: you said plainly that you couldn't run the full --all-targets
command locally because the host is Windows and the integration tests are
Unix-only. That's the right call and the right thing to write down — it's
accurate (the crate is Unix-only for now, see #149), and CI covered it on Linux
and macOS.

Merging. Thanks again.