fix(vm): handle Lua 5.3 hex and string coercion for bitwise ops

[davydog187]

Fix bitwise.lua failing assertion

Plan: .agents/plans/A5-bitwise-suite.md
Closes #166

Goal

Make bitwise.lua pass the official Lua 5.3 suite. Pre-A5, it failed
immediately on assert(0xFFFFFFFFFFFFFFFF == -1) (line 16). Post-A5, it
runs cleanly through line 270; the final third (lines 274–328) is blocked
on math.fmod, deferred to A5a.

Success criteria

• mix test passes (1382 → 1394, no regressions, +12 from new
  test/lua/vm/bitwise_test.exs)
• Each fixed assertion has a corresponding unit test in
  test/lua/vm/bitwise_test.exs (12 tests, 0 failures).
• bitwise.lua passes end-to-end. Partial — lines 1–270 now
  pass; final third deferred to A5a (needs math.fmod).

Changes

 .agents/plans/A5-bitwise-suite.md |  2 +-
 lib/lua/lexer.ex                  | 15 ++++++-
 lib/lua/vm/executor.ex            | 24 ++++++++--
 lib/lua/vm/stdlib.ex              | 47 +++++++++++++++++--
 lib/lua/vm/value.ex               | 95 +++++++++++++++++++++++++++++++++------
 test/lua/vm/bitwise_test.exs      | 77 +++++++++++++++++++++++++++++++
 6 files changed, 238 insertions(+), 22 deletions(-)

Four narrow fixes, one new follow-up plan:

1. Lua.Lexer.scan_hex_number — wrap parsed hex integer literal to
   signed 64-bit per Lua 5.3 §3.1 (0xFFFFFFFFFFFFFFFF → -1). Inlined
   the wrap (lexer doesn't depend on Lua.VM.Numeric).
2. Lua.VM.Value.parse_number — apply the same wrap to hex int
   strings, accept a leading -/+ sign, and add hex-float string
   parsing ("0xAA.0", "0x1.8p3").
3. Lua.VM.Executor.to_integer! — for bitwise operands, only accept
   floats that represent an integer exactly and fit in the signed
   64-bit range, per §3.4.3. Previously did unconditional trunc/1.
4. Lua.VM.Stdlib.lua_require — consult package.preload[name]
   before the filesystem search. bitwise.lua registers its bit32
   shim via package.preload.bit32 = function () ... end and then
   require'bit32'. Did not introduce a full
   package.searchers table — that's separate.

Discoveries

A0 covered integer arithmetic wrapping but not literal lexing, string
coercion, or float→int rules. The four fixes above are the minimal set
that gets bitwise.lua from immediate-fail to running through line 270.

A new plan A5a (A5a-bitwise-suite-math-fmod.md) covers the
remaining gap: math.fmod, used at lines 278–279 to numerically verify
bit32.lshift. With math.fmod in place, bitwise.lua is expected to
pass end-to-end (line-by-line bisect found no other gaps after line 278).

Verification

mix format
mix compile --warnings-as-errors
mix test          # 52 doctests, 45 properties, 1394 tests, 0 failures, 32 skipped
mix test --only lua53  # 29 tests, 0 failures, 25 skipped
mix test test/lua/vm/bitwise_test.exs  # 12 tests, 0 failures

End-to-end run of bitwise.lua (proves lines 1–270 pass):

$ 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
+
** (Lua.RuntimeException) ... attempt to call a nil value  # math.fmod, A5a

Out of scope (intentional)

• Architectural bitwise changes (covered in A0).
• A full package.searchers table.
• Other missing math.* functions, including math.fmod itself
  (deferred to A5a).
• Decimal integer literal overflow handling (bitwise.lua only exercises
  hex).