Function bucketGiftsThatGiveGoal

Round a raw campaign fundraising goal down to the highest accepted milestone tier (see GIFTS_THAT_GIVE_MILESTONE_GOALS).

• Nullish input returns null (nothing to bucket).
• Goals below the lowest tier ($100) return null — the campaign hasn't unlocked any tier yet, so consumers should skip the fetch (useGiftsThatGiveMilestones auto-disables on null).
• Truncates (does not floor) the raw goal so 99.99 → 99 and -100.5 → -100, matching the historical ${x}.split(".")[0] behavior used by the overlay ladder this helper replaces.