perf(stdlib): use io_lib.format for string.format float conversion

[davydog187]

Context

string.format with many specifiers is 1.85× slower than luerl (n=1000:
7.42ms vs 4.00ms in benchmarks/string_format.exs). The dominant per-call
cost is format_spec_float/2 at lib/lua/vm/stdlib/string.ex:454-501, which
calls :erlang.float_to_binary and then post-processes through
String.split, String.pad_trailing, and string interpolation in
expand_float/2.

Luerl (deps/luerl/src/luerl_lib_string_format.erl:248-250) delegates the
entire conversion to io_lib:format/2, handling precision, sign, and
rounding in a single native Erlang call.

Change

lib/lua/vm/stdlib/string.ex:454-501 (format_spec_float/2) and likely
format_spec_scientific/3 at line 503.

• Replace
  :erlang.float_to_binary([{:decimals, P}, :compact]) + expand_float
  with a direct :io_lib.format(~c\"%.*f\", [precision, val]).
• Apply the same pattern for %e and %g.
• Delete expand_float/2.

Compatibility check (required)

Confirm rounding matches C Lua across:

• test/lua/vm/stdlib/string_test.exs (all existing format cases)
• Lua 5.3 suite tests touching format/literals

Add unit tests around: precision-0 rounding (sign matters at boundaries),
large precision, inf/nan formatting.

Verification

• mix test (full suite)
• mix run benchmarks/string_format.exs

Expected

Closes the remainder of the spec-heavy gap. May flip us ahead of luerl on
many specs.