index.db: regenerate with the fixed crawler (cppreference /cpp)

Rebuilt the shipped index against cppreference.com's current /cpp URL
structure. Net effect vs the previous DB:

  cppreference: 6027 -> 6306 pages (now includes C++20/23 pages such as
                std::format, std::ranges::*, std::chrono calendar types)
  cppreference: 54822 -> 26347 keywords
  cplusplus:    unchanged (3027 pages / 5580 keywords)

The keyword drop is a cleanup, not a regression: the old DB was produced
by an older crawler that exploded allocator typedefs into malformed
aliases like 'const_pointer::const_pointer::rebind::const_pointer'
(~28k junk rows). All real lookups (containers, member functions,
typedefs) resolve at parity or better, and every URL now points at /cpp.
File shrinks 6.5MB -> 3.1MB after VACUUM.

Author: aitjcize