Implement IntoIterator for &mut LinearMap

[anyingiit]

Description

&LinearMap implements IntoIterator, but &mut LinearMap does not, so for (k, v) in &mut map { ... } fails to compile even though LinearMap::iter_mut() already exists.

This adds impl IntoIterator for &'a mut LinearMapInner<K, V, S> (mirroring the existing &'a LinearMapInner impl), returning the existing IterMut<'a, K, V>. Because it is implemented on LinearMapInner<K, V, S> with S: LinearMapStorage + ?Sized, it covers both &mut LinearMap<K, V, N> and &mut LinearMapView<K, V>. A unit test (linear_map::test::into_iter_mut) exercises both; it fails to compile without the impl. A CHANGELOG entry is added under Unreleased.

Motivation / disclosure: I had some spare AI-assistant quota (Claude Code) and am using it to try to
help projects with open good-first-issues. The change was prepared with Claude Code and verified as listed
below. If it doesn't fit, isn't up to your bar, or you'd simply rather not take it — please feel free to
close it, no hard feelings at all 🙂

Related issue

Closes #492

Checklist

• Tests pass locally (cargo test --lib → 250 passed; cargo test --doc --features alloc → 197 passed; new test fails to compile without the fix and passes with it; cargo clippy --all-targets --features "alloc,defmt,portable-atomic-critical-section,serde,ufmt,bytes,zeroize,embedded-io-v0.7" (host target) → no warnings; cargo +nightly fmt --all -- --check → clean)
• CHANGELOG.md is updated (if applicable) — added an entry under [Unreleased]
• Documentation is updated (if applicable) — n/a (trait impl, shows up in rustdoc automatically)

[sgued]

Just FIY these kinds of PR sent over the fence for the maintainers to review is pretty rude, especially using a AI generated PR description full of obvious and duplicate text.

The current code of conduct doesn't ban this, but it will likely be the case in the future.

[zeenix]

Just FIY these kinds of PR sent over the fence for the maintainers to review is pretty rude, especially using a AI generated PR description full of obvious and duplicate text.

You're likely wasting your breath. I doubt a person is looking at this. :)

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Anyway, I looked at the changes so that you don't have to and they seemed fine and trivial. Hence the approval. I didn't realize there was a conflict after I enabled auto-merge. Given that the contributor didn't bother updating the PR, gives me even more confidence that no human was involved directly here.