Return the parsed value by move from from_cbor() and friends

[nlohmann]

Part of the series that makes the binary readers non-recursive (#5104); this one stands on its own.

What

The binary entry points end with

return res ? result : basic_json(value_t::discarded);

The conditional operator's second operand is an lvalue, so this is not a case where the return value can be elided or implicitly moved from. Every successful from_cbor, from_msgpack, from_ubjson, from_bjdata and from_bson call deep-copies the value it just parsed, and then destroys the original.

Why it matters

basic_json's copy constructor walks the whole value, so the cost is proportional to the document. Parsing a 2 MB CBOR document with 60,000 objects, median of 25 runs, clang 17 -O3:

	before	after
from_cbor	26.99 ms	14.65 ms
from_msgpack	26.82 ms	14.82 ms

There is a second reason. The copy constructor recurses once per nesting level, so it is also a stack-overflow path on the return side, on a value the reader has already accepted. That is currently masked because the readers themselves recurse and overflow first, but it has to be fixed for making them iterative to have any effect — see #5104 and the rest of this stack.

API impact

No breaking changes. No signature, no return type and no observable behaviour changes; the parsed value is simply not copied on its way out. It is not used again after the return expression is evaluated.

Checklist

• The changes are described in detail, both the what and why.
• If applicable, an existing issue is referenced.
• The Code coverage remained at 100%. A test case for every new line of code.
• If applicable, the documentation is updated.
• The source code is amalgamated by running make amalgamate.

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