Clarify CastsAttributes::set() phpdoc to accept TGet; recommend symmetric casts

[andreas-pms]

Problem

When a custom cast returns an object in get(), Eloquent may later pass that same runtime value into set() (e.g. after reading and re-assigning). Without acknowledging this, casts that accept only “assignment types” (TSet) can fail in common flows like:

$a = $model->attr;   // get(): returns VO (runtime), possibly cached
$model->attr = $a;   // reassign same VO
$model->save();      // set() receives the VO -> TypeError if set() expects only TSet

This is fundamentally about assignment symmetry: developers reasonably expect $a = $a = $b to be safe.

Rationale

• Matches real call paths where set() can receive the runtime value produced by get().
• Prevents surprising TypeErrors in userland casts after read→reassign→save cycles.
• Encourages symmetric casts (TGet == TSet) so assignments remain intuitive.

No runtime behavior changes; this is a phpdoc clarification that improves developer expectations and static analysis.