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Replace snprintf with a branch-free writer for \uXXXX escapes (#5235)

* Replace snprintf with a branch-free writer for \uXXXX escapes

dump_escaped called std::snprintf(..., "\u%04x", ...) once per escaped
code point in the string serialization hot path. snprintf re-parses
the format string and pulls in locale/printf machinery on every call,
which is far heavier than the fixed 6-/12-byte output warrants. This
is hot for any string containing control characters, and for all
non-ASCII text when ensure_ascii is set.

Replace it with write_u_escape, a small helper that writes the escape
directly into string_buffer via a nibble-to-hex lookup table, mirroring
the existing hand-rolled dump_integer fast path in the same file.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix clang-tidy avoid-c-arrays warning in write_u_escape

Use a const char* rather than a char[] lookup table, matching the
existing hex_bytes helper in the same file.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* :recycle: adjust write_u_escape signature

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

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Signed-off-by: Niels Lohmann <mail@nlohmann.me>

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