[Yaki-Blake]

Closes #580.

Adds rotating_datetime_file_sink: rotates on size like rotating_file_sink, but each rotated file is renamed to a name produced by a FileNameCalc template argument -- the same convention daily_file_sink/hourly_file_sink already use, so one calculator can be shared by all three.

logs/app.log  =>  logs/app-2026-10-23_10-56-25.log
logs/app.log                                          (reopened, keeps growing)

#include "spdlog/sinks/rotating_datetime_file_sink.h"

auto logger = spdlog::rotating_datetime_logger_mt(
    "logger", "logs/app.log", 5 * 1024 * 1024, /*max_files=*/10);

The active file keeps a fixed name on purpose: file_helper opens it in append mode, so a restarted process continues the same file and max_size keeps being enforced. A timestamp in the active name would start a new file on every run (measured: two runs give 2 files instead of 1), silently defeating max_size/max_files across restarts. That is what rotate_on_open is for.

API mirrors rotating_file_sink (set_max_size, set_max_files, set_buffer_size, rotate_now, rotate_on_open). Additive only -- no existing header or source is modified. 10 Catch2 cases added; full suite passes with MSVC 19.51 and MinGW g++ 8.1, header-only and compiled lib, no warnings.

[gabime]

Thanks. Not sure I want to add whole new sink for this rarely required feature. Closing for now