Drop the C shim for float formatting now that jsonpull is C++

json_print_one() reached fpfmt through dtoa_shortest(), a C-linkage
wrapper in text.cpp that strdup()ed the formatted string so the C
jsonpull could take it and free() it. jsonpull has been C++ since 4f26211,
so it can append the std::string directly and skip the copy, the
allocation, and the free.

That was the shim's only caller, so fpfmt/fpfmt.h goes away with it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014wJRAuhMninQE4wK2TUfuZ

CHANGELOG.md

@@ -9,7 +9,9 @@
   final digit (`9.823748927348929e+55` becomes `9.823748927348928e+55`). Every
   such value still parses back to exactly the same double, so this changes only
   the spelling, never the number. `make fpfmt-bench` rebuilds the head-to-head
-  comparison against the old implementation.
+  comparison against the old implementation. `jsonpull`, now that it is C++,
+  calls `fpfmt::dtoa()` directly rather than through the `strdup`ing C shim
+  that `milo/milo.h` used to declare.
 
 # 2.82.0