Log invalid use of _cf_ table prefix

[maxmcd]

Add warning logs to detect SQL identifiers using mixed-case cf prefix variants (e.g. CF, Cf) and FTS5 virtual tables using any-case cf prefix.

[ask-bonk]

I'm Bonk, and I've done a quick review of your PR.

This PR adds warning logs to detect mixed-case _cf_ prefix usage in SQL identifiers and FTS5 virtual tables.

Issues

1. [High] <strings.h> included unconditionally — breaks Windows/MSVC build.
   strncasecmp is a POSIX function declared in <strings.h>, which doesn't exist on MSVC. Other files in the codebase handle this: form-data.c++ guards the include with #if !_MSC_VER, and sqlite.c++ uses #define strncasecmp _strnicmp. The new code in sql.c++ does neither.

2. [Low] Log messages don't include the offending identifier.
   Since LOG_WARNING_PERIODICALLY is rate-limited to once per hour per call site, including the actual name would be much more useful for debugging when these fire in production. Both call sites have the name readily available.

[ask-bonk]

Review posted successfully on PR #6448. Here's a summary of the findings:

1. [High] <strings.h> portability issue — The POSIX header is included unconditionally in sql.c++, which will break Windows/MSVC builds. Suggested a platform-guarded fix matching the pattern used in sqlite.c++.

2. [Low] Log messages missing identifier names — Both LOG_WARNING_PERIODICALLY call sites omit the offending SQL identifier/table name. Since these are rate-limited to once per hour, including the name would significantly help debugging.

All three issues have concrete suggestion comments on the PR.

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