Write the manual for the person using the desktop

docs/ is engineering artifacts -- design specs, plans, an upstream
ledger -- and none of it is written for the person who has to live
here. Five chapters that are: coming from another desktop, the
keyboard, windows and workspaces, when something breaks, and making it
yours.

Rendered inside Settings rather than opened in a browser, so a chapter
that says "the Displays page reverts after fifteen seconds" is one
click from the Displays page. Qt's markdown renderer does the work;
one chapter per Text, because Text has an implicit texture size limit
and a document long enough to hit it goes blank rather than
complaining.

The chapters live beside the shell in manual/ rather than at the
repository root, which departs from the plan. The reason is the path:
the shell finds them through Quickshell.shellDir, which is correct
wherever the repository is, whereas walking upward out of the shell
directory is only correct by accident. The contract fails that pattern
if it comes back.

The contract also pins the set both ways -- every chapter listed
exists, every chapter that exists is listed -- because a renamed file
shows an error card where a chapter should be, which reads as a broken
manual rather than as a moved file.

README.md

@@ -108,12 +108,13 @@ setup/
   packages/     One package per line; extras/ holds the optional categories
   scripts/      Run in order by ./install
 tests/          Contracts. See below
+  manual/       The user manual, rendered in Settings
 docs/           Settings reference, and the design specs behind the work
 ```
 
 ## Tests
 
-142 of them, under `tests/`. Run the lot, or a subset by pattern:
+143 of them, under `tests/`. Run the lot, or a subset by pattern:
 
 ```sh
 panama test           # everything