Support integer parts-per-second time unit

[petermm]

Add support for positive integer time units ("parts per second") in erlang:monotonic_time/1, erlang:system_time/1, and calendar:system_time_to_universal_time/2.

Used in elixir DynamicSupervisor - completes the timeunit support.

Restrict integer-unit handling to int64 inputs in the affected NIF paths. Use checked int64 decomposition for monotonic/system time conversion to avoid signed overflow in intermediate arithmetic. For calendar integer units, floor negative fractional values to whole seconds before converting to UTC.

Add focused Erlang tests for integer-unit parity, badarg on non-positive integer units, and negative fractional calendar conversion for integer units.

These changes are made under both the "Apache 2.0" and the "GNU Lesser General
Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later

[bettio]

tests are not working on the BEAM, let's fix this first.

log from run-tests:

datetime:OK
Raised {badmatch,fail}, stacktrace:
[{test_system_time,test_bad_integer_unit_universal_time,0,
                   [{file,"/__w/AtomVM/AtomVM/tests/erlang_tests/test_system_time.erl"},
                    {line,259}]},
 {test_system_time,start,0,
                   [{file,"/__w/AtomVM/AtomVM/tests/erlang_tests/test_system_time.erl"},
                    {line,45}]},
 {erl_eval,do_apply,7,[{file,"erl_eval.erl"},{line,915}]},
 {erl_eval,try_clauses,10,[{file,"erl_eval.erl"},{line,1233}]},
 {erl_eval,expr,6,[{file,"erl_eval.erl"},{line,663}]},
 {erl_eval,exprs,6,[{file,"erl_eval.erl"},{line,271}]},
 {init,start_it,1,[]},
 {init,start_em,1,[]}]
test_system_time:
test_system_time:FAILED

[petermm]

thank you, shall blame the CI blunder on it being backed up;-)

let me know if a squash is needed.

[bettio]

Just a couple of minor fixes. Feel free to cleanup PR history and push it for merge.

[petermm]

Hit some complexities - namely OTP only support "extreme" values in calendar since OTP29 - erlang/otp#10449

then further complexities around negative numbers, what esp32/newlib uses, and some theoretical issues for years +-134 millions and beyond on some 32bit system :/

time is always fun, should be ready now.