Calcium in various animal shells is almost completely insoluble in water, except in acidic solutions. That's why limestone exists. Limestone consists of layer after layer of those diatom shells. They have been sitting there for millions of years, after spending many years on the bottom of seas. Adding any kind of shell to plant medium will not provide enough extra calcium to matter to the plants.
Calcium quickly binds to many ions commonly found in water, and forms almost-insoluble salts. That is why people use commercial calcium supplements, and mix them in pure water (rain, distilled, reverse osmosis.) They are in forms in which the calcium is soluble, so long as you don't dissolve them in low-pH water that already has a lot of insoluble calcium compounds.
You can look up your water quality on the Web page of your local water supplier. Look for their Annual Water Quality Report. It will show whether there is much calcium in your water. If there is any, acidifying your tap water to the 5.5-6.5 range will make plenty of calcium available to your plants. I don't recall where Seattle gets its water; if it's from wells, there is a good chance your have plenty of calcium available.
Edit:
Here's the 2023 Seattle water quality report. Unfortunately and regrettably, it only provides information about toxins, and not the things important to gardeners. You could telephone them. Ask what is the pH, total dissolved solids, and calcium content. They measure those but didn't report them.
Diatomaceous earth for calcium?
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