Signals, computed properties and other reactivity patterns in Hologram

[bartblast]

@venkatd

I would suggest rephrase as:
”The purpose is reactivity - enabling automatic re-rendering when values change”

I deleted all mentions of implementation details to focus on the ideal API/DX.

(If we talk about memoization and such things in the problem statement, then we we allowing the implementation details to leak into the core design.)

Anyway, just a suggestion

I rephrased it to focus on “efficient derived value management” rather than “reactivity” - since Hologram already has component reactivity built-in, what this feature actually adds is smart management of derived values during those re-renders.