[D-1] Decide what a revision comparison looks like

[liangricky7]

Blocked by: — · Blocks: D-3 (#32) (if it shows a diff), and the frontend for Epic 3

(Was sprint 1 ticket 10.) A comparison must make added, removed and moved text distinguishable, and the palette rules out the obvious answer. Amber is reserved for save conflicts only, and the accent colour is never used to show state, so red/green is out.

The backend's comparison API (T2-4, #13) returns the data either way. This ticket decides only how it looks.

• Choose a neutral treatment (background weight, a mark in the margin, a symbol — the same approach the permission badge uses), or explicitly allow an exception to the colour rule and write down why
• It works in light and dark mode, and survives greyscale
• It never relies on colour alone
• Record the decision in DESIGN.md §8, alongside the other components

Needs a human design call, not an implementation.

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Source: docs/frontend-tickets.md