Remove duplicated and refusal-only convertor specializations

Removes boilerplate from ext/convertors (net −172 lines): DevVarCharArray element conversion reuses the DEV_UCHAR converter, 19 refusal-only template specializations fold into if constexpr guards in their primary templates, and the two duplicated MultiAttrProp casters give way to per-member assign_attr_prop deduction. Reusing the DEV_UCHAR converter also range-checks DevVarCharArray elements, so -1 and 256 now raise instead of wrapping silently to 255 and 0 — the one behaviour change, covered by a news entry.

Stacked on !1051 (merged); targets convertors-bugfixes until that merges.

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