Do you consider unhealthily browned calyxes as first indicators of developing bud rot?

I don't quite understand what you're worried about; nothing abnormal stands out. The calyx and its pistils are simply withering; it's mostly just an indicator of the plant's sex. Good luck, my friend.

-- i misread a bit earlier That upper calyx in red circle is probably just turning a bit purple due to ripeness... Doesn't necessarily mean the buds will be purple, but possible. No sign of rot.

In the red circle, it’s just a small spider: leave it alone, it's a beneficial predator that eats harmful pests and won't damage the plant.In the green circles, you need to double-check closely. Look at them up close: if they are female calyxes with white hairs (pistils) growing out of the tip, it’s completely normal, they just swelled up. However, if they look like closed, egg-shaped or 'banana-shaped' pods with no hairs, the plant is showing male pollen sacs (hermaphroditism). If so, they should be carefully plucked with wet tweezers to prevent pollen from spreading."

That calyx looks genetic, for me first sign of bud rot is stripy purple main stem, then it’s either white or grey fluff on the buds. Different types of bud rot tbh. Sometimes you get leaves that come out easily just or pulling. If you have the mushy bud you can chop it out and save it. If you have white fluff the entire crop is comprised. How do I know? I’ve had it…

Hairs turn rusty colored. That's part of senescence. They die, and turn color. Preflowers are the oldest calyxes on the plant, so those often turn color first. When you see a bunch of sugar leaves in a cola die for seemingly no reason, that's a bad sign of something lurking underneath.. Gently pull buds apart without damaging it, and inspect for white or grey fuzz underneath. Bortytis is a fungus, so you see 'hyphae' - the thread-like fuzz. Unusual patches of discolored buds (calyxes, not hairs) is bad too. Whether it's bortytis or not, kinda irrelevant. Infections are bad... "Unngggh! Fire, bad!" - Frankenstein's monster.

I also agree with organoman

No..........absolutely not.............not related at all. Pre-flowers growing on the stem often/usually look like this before harvest as they are much, much older than the actual flowers. "Bud rot" starts deep inside dense buds, not on exposed single calyxes..............you have nothing to be worried about.