Cache sub-encoder isos in ScalarTypeEncoder

[veewee]

Summary

Captures the int/float/string/bool isos once in iso() instead of creating new IntTypeEncoder() etc. and calling ->iso($context) on every encode invocation.

Impact

<1% (negligible). Each sub-encoder's iso() is trivially cheap. The value is consistency: iso() should capture its dependencies eagerly, not rebuild them per call.

Test plan

• Full test suite passes (549 tests)
• Psalm clean
• CS fixer clean

[veewee]

Closing: the 4 sub-encoder isos are only used in the to() direction, while decode (the hot path) doesn't use them at all. This means we eagerly allocate 4 unused isos on every decode call. No measurable benefit, slight waste on decode.