feat(stdlib): implement math.fmod for bitwise.lua bit32 verification

[davydog187]

Add math.fmod for bitwise.lua bit32 verification

Plan: .agents/plans/A5a-bitwise-suite-math-fmod.md

Goal

Implement math.fmod so bitwise.lua can finish its bit32-library
verification block (line 278 onward). This was the last remaining gap
discovered while shipping A5.

Success criteria

• math.fmod(x, y) works per Lua 5.3 §6.7: "Returns the remainder
  of the division of x by y that rounds the quotient towards zero."
  Truncated-quotient remainder via Kernel.rem/2 for integers and
  :math.fmod/2 for floats.
• math.fmod of two integers returns an integer; otherwise returns
  a float. Verified by math.fmod returns integer remainder and
  math.fmod with mixed int/float returns a float tests.
• mix test passes, no regressions. 1394 → 1402 tests, 0 failures
  (8 new tests, 1 fewer skipped — bitwise.lua moved from
  @skipped_tests to @ready_tests).
• Unit tests in test/lua/vm/stdlib/math_test.exs cover
  integer/integer, integer/float, float/float, negative dividend,
  mininteger / -1 overflow guard, integer divide-by-zero (raises
  "bad argument Add CI #2 ... (zero)"), float divide-by-zero (raises — see
  Discoveries), non-number argument errors, and missing-argument
  errors.
• bitwise.lua passes end-to-end. Verified by running the file
  directly and by adding it to @ready_tests (mix test --only lua53 passes 5 ready files now).

Changes

 .agents/plans/A5a-bitwise-suite-math-fmod.md |  16 +++-
 lib/lua/vm/stdlib/math.ex                    |  69 +++++++++++++++++
 test/lua/vm/stdlib/math_test.exs             | 106 +++++++++++++++++++++++++++
 test/lua53_suite_test.exs                    |   2 +-
 4 files changed, 190 insertions(+), 3 deletions(-)

Discoveries

• Lua 5.3 returns NaN for math.fmod(x, 0.0) when either argument is a
  float. The BEAM has no NaN value, and the rest of this VM raises on
  zero-divisor float arithmetic (see safe_divide in
  lib/lua/vm/executor.ex:1690). For consistency we raise bad argument #2 to 'math.fmod' (zero) for both integer and float zero divisors.
  Documented inline in lib/lua/vm/stdlib/math.ex.
• Integer math.fmod(mininteger, -1) is short-circuited to 0 to avoid
  an overflow trap (matches the C implementation in lmathlib.c's
  special case for d == -1).
• Moved bitwise.lua from @skipped_tests to @ready_tests in
  test/lua53_suite_test.exs, locking in the suite-count gain.
• No other math.* gaps surfaced while wiring fmod in. math.modf and
  math.atan2 are not exercised by bitwise.lua.

Verification

mix format
mix compile --warnings-as-errors
mix test                                 # 1402 tests, 0 failures, 31 skipped
mix test test/lua/vm/stdlib/math_test.exs  # 25 tests, 0 failures
mix run --no-mix-exs -e 'Code.require_file("test/support/lua_test_case.ex"); Lua.TestCase.run_lua_file("test/lua53_tests/bitwise.lua")'
# → testing bitwise operations / + / testing bitwise library / + / + / OK
mix test --only lua53                    # 29 tests, 0 failures, 24 skipped

Out of scope (intentional)

• Other missing math.* functions (math.modf, math.atan2, etc.)
  unless also exercised by bitwise.lua. None surfaced.
• A package.searchers table — separate concern from the package.preload
  hook added in A5.