Thanks to @CoderMike's comment pointing me to the official Pi forum, camera section it was then easy to figure this out by typing "filter" in the search box specific for the camera topic within the forum.
I've found that this is the famous infrared filter (never seen one before!) that almost all cameras intended for realistic viewing have, since silicon is sensitive past 1000 nm while our eyes cut off at about 700 nm.
several posts below on the April 2022 Raspberry Pi HQ Camera Grainy/Noisy Image page indicate that this blueish rectangular infrared filter becomes mottled and degraded optically. One user mentioned working outdoors in sunlight but another mentioned that like my situation, it was simply stored.
The only fix seems to be to remove the filter itself.
Arducam's Raspberry Pi High Quality Camera 12MP: 5 Things to Know Before You Buy links to the official Raspberry Pi documentation's Raspberry Pi HQ Camera Filter Removal; the first two steps of which are:
- Work in a clean and dust-free environment, as the sensor will be exposed to the air.
- Unscrew the two 1.5 mm hex lock keys on the underside of the main circuit board. Be careful not to let the washers roll away. There is a gasket of slightly sticky material between the housing and PCB which will require some force to separate...
After removing the two hex screws from the circuit board side, we successfully separated the circuit board from the big black ring stuck to it by inserting a tiny flat screwdriver under one edge and gently turning it.
The "slightly sticky material" gasket gave way easily and it was not difficult to gently pop the infrared filter off with a tiny screw driver at one corner, accessible by the machined clearances in each corner of the recess in which it sits.
The camera now works great but of course the color is no longer realistic as the red channel is dominated by infrared.
above: Cropped bit of an image from Raspberry Pi HQ Camera Filter Removal showing the recessed corner where one can get a bit of leverage to remove the filter. In this image they've replaced it with a piece of high quality optical plastic in order to protect the sensor surface from dust. below: remove the two screws from the PC board side first, then separate the big black ring from the circuit board by applying "some force" to break the "slightly sticky material" gasket.
After removing the filter you can reseat it and replace those two hex screws. *If you are clever you'll remember to take care to rotate it so that the standard camera mount is in the right direction and oriented so that when you mount it the CCD sensor won't be slightly rotated (tilted) :-)
Source: Raspberry Pi High Quality Camera 12MP: 5 Things to Know Before You Buy
above: detailed view of the removed IR filter; the spots are visible at the top where the background is dark. below: the removed IR filter.