Add insert() and erase() to editable json_documents

[nlohmann]

Part of the stack for the zero-copy view (#5295).

Summary

Editable documents can now change the structure of arrays and objects:

• insert(array, index, value) inserts before an element (index <= size()) and returns a view of the new element;
• erase(object, key) removes all members with the key and returns their number;
• erase(array, index) removes an element;
• erase(json_pointer) removes the member or element a pointer names and returns the number of removed values.

A view of an erased value keeps its last value. Views of other values keep referring to them when elements move.

Errors are those of basic_json: type_error.307/309 and out_of_range.401/403/405.

bench_edit.cpp joins the comparison harness. It parses, applies the same logical edits with each library's own API (a handful at fixed places, or one in every record), and serializes; all outputs must describe the same value. compare.py builds and runs it with the other two programs.

Benchmarks

bench_edit at this PR, on Apple M1, best of 25 rounds. Each run parses, applies the edits, and serializes. json_view's time is the base (1.00×); higher factors are slower:

document	edits	json_view	yyjson	Boost.JSON	json
twitter	patch	463 µs	1.08×	2.91×	4.95×
citm_catalog	patch	756 µs	1.21×	2.58×	5.91×
canada	patch	10.7 ms	0.35×	0.61×	1.67×
jeopardy	update	71.4 ms	1.20×	2.13×	3.70×
status (small)	patch	2.78 µs	0.99×	2.30×	4.15×
rpc (tiny)	patch	0.40 µs	0.87×	1.01×	2.39×

Serializing (dump()) takes about half of json_view's time on twitter and most of it on canada. It gets faster later in the stack, in the PRs that add the compact writer, Żmij, and floats written from their digits. At the top of the stack (same machine and method, patch):

• twitter 309 µs (yyjson 1.65×), citm_catalog 567 µs;
• canada 5.69 ms, still behind yyjson (3.86 ms), whose float writer is faster.

x86-64 numbers are still missing.

Tests

• The differential test also inserts and erases members and elements, directly and through JSON pointers.
• The errors.
• Views across inserts and erasures.
• Duplicate keys and large objects.

Documentation

• API pages for insert and erase, with examples.
• The editing section of the feature page names them.

Public API

No breaking change. The new members exist only for editable documents.

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Written by Claude Code.

🤖 Generated with Claude Code

[nlohmann]

The zero-copy view stack (#5295) was rebased onto develop and regrouped from 20 PRs into 10, one squashed commit each. The changes of this PR are now in #5630 (the bench_edit benchmark went to #5627); its review comments stay here for reference.

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Written by Claude Code.