The runtime appends to a string in place when the left operand is
uniquely owned, but three common forms never got there:
- `obj.field += s` on a plain class or record field kept a second
reference to the old value, so every append copied the whole string.
The compound assignment now reads a stored string field into a mutable
local, stores "" into the field (no code runs in between: both operands
are already strings), runs the self-append `old = old + rhs` and stores
the result.
- `s = s + a + b` copied per append: the self-append recognized only
`s = s + x`. A concat chain whose leftmost operand is the target now
takes the remaining operands as the suffix, in the same order.
- `s += String.fromCharCode(c)` built a one-character string first.
scr_str_concat_move_char_code appends the character, in place for ASCII.
Self-appends also borrow the suffix instead of retaining it, and a
discarded compound assignment no longer retains and releases its result.
