fix(cron): fix scheduling bugs and add destroy method

[hakanesnn]

Fixes

• getMaxDaysInMonth: Fixed off-by-one error by using day=0. Previously returned 30 for January, 27 for February, etc.
• getNextTime logic: Rewritten to scan forward for the next execution time. This fixes:

1. Hour ranges (e.g., 0 17-23 * * *) firing only once and stopping (Issue: [Bug] Cron - Day ahead or more cron doesn't run #77).
2. Future-dated crons (e.g., 0 0 01 01 *) failing to schedule if the target date is not today (Issue: [Bug] Cron not working properly #95).

Improvements

• task:destroy(): New method to stop tasks and remove them from the registry, ensuring they can be garbage collected.
• Task ID management: Switched to an incrementing counter instead of #tasks+1 to prevent ID collisions when tasks are destroyed.
• manualStop flag: Prevents the midnight reschedule job from restarting tasks that were intentionally stopped by the user.
• Code Cleanup: Removed ~70 lines of redundant code in getAbsoluteNextTime by delegating logic to the updated getNextTime.

Test Code

-- Test 1: Hour range - uses current hour so minute overflow always triggers the bug
-- OLD: returns current hour's :00 (already past), task stops
-- NEW: returns next hour's :00 within the range
local currentHour = tonumber(os.date('%H'))
local rangeEnd = math.min(currentHour + 2, 23)
local hourExpr = ('0 %d-%d * * *'):format(currentHour, rangeEnd)
local t1 = lib.cron.new(hourExpr, function() end, { debug = true })
local next1 = t1:getNextTime()
print(('[Test 1 - Hour Range] expression: %s | isActive: %s | nextTime: %s'):format(
    hourExpr,
    tostring(t1.isActive),
    next1 and os.date('%c', next1) or 'nil'
))
t1:stop()

-- Test 2: Future day - schedules for tomorrow
-- OLD: returns nil because day != today
-- NEW: returns tomorrow's date
local tomorrow = os.date('*t', os.time() + 86400)
local expr = ('0 0 %d %d *'):format(tomorrow.day, tomorrow.month)
local t2 = lib.cron.new(expr, function() end, { debug = true })
local next2 = t2:getNextTime()
print(('[Test 2 - Future Day] expression: %s | isActive: %s | nextTime: %s'):format(
    expr,
    tostring(t2.isActive),
    next2 and os.date('%c', next2) or 'nil'
))
t2:stop()

-- Test 3: getMaxDaysInMonth - verifies day=0 fix
-- day=-1 returns second-to-last day, day=0 returns correct last day
local jan_old = os.date('*t', os.time({ year = 2026, month = 2, day = -1 })).day
local feb_old = os.date('*t', os.time({ year = 2026, month = 3, day = -1 })).day
local apr_old = os.date('*t', os.time({ year = 2026, month = 5, day = -1 })).day
local jan_new = os.date('*t', os.time({ year = 2026, month = 2, day = 0 })).day
local feb_new = os.date('*t', os.time({ year = 2026, month = 3, day = 0 })).day
local apr_new = os.date('*t', os.time({ year = 2026, month = 5, day = 0 })).day
print(('[Test 3 - Max Days] day=-1: Jan=%d Feb=%d Apr=%d | day=0: Jan=%d Feb=%d Apr=%d'):format(
    jan_old, feb_old, apr_old, jan_new, feb_new, apr_new
))

-- Test 4: Last day of month via L expression
-- OLD: nil (day != today + off-by-one)
-- NEW: correct last day of current month
local tL = lib.cron.new('0 0 L * *', function() end, { debug = true })
local nextL = tL:getNextTime()
print(('[Test 4 - Last Day (L)] nextTime: %s'):format(
    nextL and os.date('%c', nextL) or 'nil'
))
tL:stop()

-- Test 5: Task ID uniqueness after destroy
-- OLD: destroy() does not exist
-- NEW: IDs never collide after destroy
if t1.destroy then
    local t3 = lib.cron.new('0 0 * * *', function() end)
    local t4 = lib.cron.new('0 0 * * *', function() end)
    local id3, id4 = t3.id, t4.id
    t3:destroy()
    local t5 = lib.cron.new('0 0 * * *', function() end)
    print(('[Test 5 - Task ID] id3=%d id4=%d id5=%d | no_collision=%s'):format(
        id3, id4, t5.id,
        tostring(t5.id ~= id3 and t5.id ~= id4)
    ))
    t5:destroy()
    t4:destroy()
else
    print('[Test 5 - Task ID] skipped (destroy not available)')
end

-- Test 6: Manual stop flag
-- OLD: manualStop does not exist
-- NEW: manualStop=true when user calls stop()
local t6 = lib.cron.new('0 0 * * *', function() end)
t6:stop()
if t6.manualStop ~= nil then
    print(('[Test 6 - Manual Stop] manualStop: %s'):format(tostring(t6.manualStop)))
    if t6.destroy then t6:destroy() end
else
    print('[Test 6 - Manual Stop] skipped (manualStop not available)')
end

Old code:

[Test 1 - Hour Range]  expression: 0 13-15 * * * | nextTime: Mon Apr  6 13:00:00 2026  (past time, task dies)
[Test 2 - Future Day]  expression: 0 0 7 4 *    | nextTime: nil
[Test 3 - Max Days]    day=-1: Jan=30 Feb=27 Apr=29 | day=0: Jan=31 Feb=28 Apr=30
[Test 4 - Last Day (L)]                             | nextTime: nil
[Test 5 - Task ID]     skipped (destroy not available)
[Test 6 - Manual Stop] skipped (manualStop not available)

New code:

[Test 1 - Hour Range]  expression: 0 14-16 * * * | nextTime: Mon Apr  6 15:00:00 2026
[Test 2 - Future Day]  expression: 0 0 7 4 *    | nextTime: Tue Apr  7 00:00:00 2026
[Test 3 - Max Days]    day=-1: Jan=30 Feb=27 Apr=29 | day=0: Jan=31 Feb=28 Apr=30
[Test 4 - Last Day (L)]                             | nextTime: Thu Apr 30 00:00:00 2026
[Test 5 - Task ID]     id3=5 id4=6 id5=7 | no_collision=true
[Test 6 - Manual Stop] manualStop: true