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refactor: use `-=`, `*=` and `/=` for self-arithmetic

Completes the compound-assignment sweep started in #3945, which covered
only `+`. 21 sites in 11 files.

Found with moongrep, one pattern per operator:

    moongrep scan --pattern '$(a:id) = $(a:id) - $(_:exp)'   # 14
    moongrep scan --pattern '$(a:id) = $(a:id) * $(_:exp)'   #  5
    moongrep scan --pattern '$(a:id) = $(a:id) / $(_:exp)'   #  2

Matching on the AST makes this precedence-safe for free, which matters
much more for `*` and `/` than it did for `+`. `x = x * a + b` parses as
`Add(Mul(x, a), b)`, so its root is `Add` and the `*` pattern simply
does not match it — rewriting it to `x *= a + b` would have changed
`(x * a) + b` into `x * (a + b)`. In #3945 the equivalent trap had to be
excluded by hand with a top-level-operator scan over the text.

The family stops here. `%=`, `&=`, `|=`, `^=`, `<<=` and `>>=` are not
MoonBit syntax — `hx &= 0x7fffffff` is a parse error — even though the
corresponding `x = x & ...` shape occurs 64 times across the tree. Those
are left as-is.

Tests pass on wasm-gc, js and native; `moon info` reports no `.mbti`
change.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

8 of 9 new or added lines in 6 files covered. (88.89%)

15789 of 17459 relevant lines covered (90.43%)

173601.36 hits per line