Story: Replace the deprecated sscanf calls

No translation unit calls a C runtime function that MSVC's STL deprecates, so the Windows clang build compiles with -Werror.

• run_document_generator.cpp parses the run date through datetime::from_iso8601_date.
• process_executable_tests.cpp reads the build timestamp without std::sscanf.
• The reporting core and platform tests still pass.

• This is the sanction the memory already states. Use ores.platform for cross-platform functionality requires a platform primitive in place of a native one wherever ores.platform has one, because Windows Clang compiles with -Werror -Wdeprecated-declarations and MSVC's STL deprecates the native call. Both sites here are that situation with std::sscanf rather than std::getenv, and both are answered by the platform primitive rather than a suppression.
• Use the platform's ISO 8601 date primitive rather than suppressing the warning. datetime::from_iso8601_date accepts the extended form and the basic form, validates the calendar date, and throws std::invalid_argument otherwise. previous_weekday hand-rolled exactly that with std::sscanf and then repeated the ok() check, so the change removes the deprecated call and a duplicate parser at once. It is also the primitive the estate already uses for ISO dates, at twelve call sites.
• Not time_utils::parse_date, which is the wider reader. That parser exists for ORE's market-data text and deliberately accepts 30.09.2025, 30-09-2025 and 30.09.25 as well as both ISO forms; its own tests pin those. The run date is required to be ISO 8601 and is refused in those words, so the wider reader would silently accept shapes the check means to reject.
• Do not add _CRT_SECURE_NO_WARNINGS outside the platform wrapper. The repository already defines it in real_environment_provider.cpp, whose whole job is to wrap getenv, and that is the pattern the getenv fix established. A reporting service and a test are not that, so both call sites lose the call instead of the warning.
• The timestamp test uses the platform's format parser. datetime::from_utc_format is the primitive for text that is not ISO 8601, such as a log line. It throws unless the whole string matches the format given, so the case proves the format by parsing it rather than by slicing fixed widths.

• The getenv calls. Those go through the platform provider already, which is the sanctioned wrapper and carries the define.
• Any other deprecated C runtime call. A sweep over projects/ for the functions MSVC's STL deprecates found no others.