Input parser rework: migrate FPType to MSTD_ENUM (reserved-keyword naming) + floating_point_type key

[97gamjak]

Part of #515. Depends on Phase 3 being complete (as precedent, not a hard
technical dependency).

Scope

Convert FPType from a plain enum class to MSTD_ENUM, and migrate the
floating_point_type key in generalInputParser.

Real gotcha, not hypothetical: the accepted input tokens are
"float"/"double" -- both reserved C++ keywords, so they cannot be
literal enumerator names. Use non-colliding identifiers (e.g. flt/dbl)
and a customParser mapping the real input tokens to them:

customParser = [](std::string_view raw) -> std::optional<FPType>
{
    if (raw == "float")  return FPType::flt;
    if (raw == "double") return FPType::dbl;
    return std::nullopt;
};

Single key -- no reason to split enum conversion and key migration into
separate PRs here.

Acceptance criteria

• "float"/"double" input tokens resolve correctly via the
  customParser alias.
• Existing integration tests for floating_point_type pass unchanged.
• New unit test covering the alias explicitly (not just the happy path).
• Old parseFloatingPointType and fpTypeFromString/string(FPType)
  deleted in this PR once unreferenced elsewhere.

Risk

Low-moderate -- the naming workaround needs to be gotten right, but blast
radius is one key.