No knowable rework

Build the final shape of what's needed now. A change is wrong if, on the day it lands, the rework is already nameable — nothing about the requirements has to change for it to need redoing or removing. One test, two failure modes:

• A corner-cut fails forward: the redo is scheduled the day it ships. Rendering Mermaid by hand-written string replacement when the vision says extensions plug into the parser; a full rebuild on every save when the vision says rendering is realtime. The tell is a known future task created to save present effort — "do X properly later" where later is guaranteed.
• Overengineering fails backward: the removal is justified the day it ships. A layout no document uses, a plugin system for imagined extensions, a config field nothing reads.

"Needed now" is judged against the design and the vision for the shape of what is built, not against the current ticket alone; whether to add new surface at all is governed by Surface area arrives with its consumer. And "final shape" is not "every feature" — a syntax set holding only what today's documents use is final-shaped, because adding an extension when a document needs one is ordinary evolution, not scheduled rework.