Remove ^= as mutation for |= and &= operators

[Copilot]

Problem

When mutating bitwise assignment operators, cargo-mutants was suggesting replacing |= with ^= (and &= with ^=). However, this mutation is uninformative for the common pattern of accumulating bits into a bitmap that starts from zero:

let base_ix = self.text.len();
self.chars |= slice.chars << base_ix;
self.chars_utf16 |= slice.chars_utf16 << base_ix;

In this code (from Zed's crates/rope/src/chunk.rs), when accumulating new set bits into a bitmap that's initially zeros, |= and ^= produce identical results. The mutation doesn't actually test anything different.

Solution

Changed the mutation patterns for bitwise assignment operators:

• &= now only mutates to |= (removed ^=)
• |= now only mutates to &= (removed ^=)
• ^= still mutates to both |= and &= (unchanged)

This makes the mutations more meaningful by ensuring they actually change the behavior in typical usage patterns.

Changes

• Updated mutation logic in src/visit.rs
• Updated documentation in book/src/mutants.md with explicit entries for bitwise assignment operators
• Added changelog entry to NEWS.md
• Updated test expectations in mutate_assign_ops test

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Reconsider replace |= with ^=</issue_title>
<issue_description>```rust
#[inline(always)]
pub fn append(&mut self, slice: ChunkSlice) {
if slice.is_empty() {
return;
};

    let base_ix = self.text.len();
    self.chars |= slice.chars << base_ix;
    self.chars_utf16 |= slice.chars_utf16 << base_ix;
    self.newlines |= slice.newlines << base_ix;
    self.tabs |= slice.tabs << base_ix;
    self.text.push_str(slice.text);
}

In this code from Zed `crates/rope/src/chunk.rs` we're accumulating new set bits into a bitmap that's initially zeros. It's probably a common pattern. Warning that the or might be xor may not be helpful enough?</issue_description>

<agent_instructions>Change the code in visit.rs so that it no longer mutates `|=` to `^=`. Update the table of patterns in book/src/. Mention it in the news. Update the tests.</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

Fixes #550

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