Demangle symbols names in fzf

When showing the user an fzf to select a trigger symbol, show demangled symbols instead of mangled ones.

Symbols specified at the command line, I think, should still be mangled to ease copy+paste between apps and to avoid forcing us to write a name mangler too.

The message we show after a user selects a symbol should print its mangled name so the user can copy+paste it into future magic-trace invocations.

Locate the fzf trigger-symbol selection flow and the message shown after a symbol is selected. Make the picker display demangled names while keeping command-line symbols and the post-selection message mangled, then verify both paths preserve copy-and-paste behavior.

Written by the indexing model from the issue text.