build: install the manual alone, into the prefix or the user datadir

The manual is the only thing udpscope installs, but the only way to get it
where `help:` resolves was a whole `cmake --install`. Two opt-in targets
now install just the set, each printing where it wrote:

- install-help goes through the install machinery into CMAKE_INSTALL_PREFIX
  (reachable via the new Help component), so a packager or a system-wide
  user points it at /usr and wraps it in sudo;
- install-help-user resolves the user data directory the way the app does
  ($XDG_DATA_HOME, else $HOME/.local/share) and copies there without root.

Neither is built by default and neither changes what `cmake --install`
does; the install() rule keeps its destination.

docs/claude/build-and-run.md

@@ -74,6 +74,28 @@ GApplication id and the install directory at once. The figures are regenerated b
 `tools/make-help-shots.sh`, which drives the app headlessly against the mock and draws the annotation
 boxes with ImageMagick; Mallard is translatable with `itstool`, but only the `C` set is authored.
 
+Because the manual is the only thing udpscope installs, it can be installed *alone* — where `help:`
+finds it, and without the `cmake --install` of a whole project — by either of two opt-in targets.
+Neither is built by default and `cmake --build build` never touches either:
+
+```sh
+cmake --build build --target install-help-user   # <data-home>/help/C/<id>/, no root
+cmake --build build --target install-help         # <CMAKE_INSTALL_PREFIX>/share/help/C/<id>/
+```
+
+`install-help-user` writes to `$XDG_DATA_HOME` when it is set and to `$HOME/.local/share` when it is
+not — the XDG Base Directory Specification's own default for the variable, and the fallback
+`help_installed_path()` uses — so it needs no root and lands the set exactly
+where `F1` and the alert look for it. `install-help` goes through the install machinery into the
+configured `CMAKE_INSTALL_PREFIX`, so a packager or a system-wide user points it at `/usr` with
+`-DCMAKE_INSTALL_PREFIX` at configure time. Neither target can install system-wide by itself: a build
+target has no more privilege than the shell that ran it, so `install-help` writes to the configured
+prefix and a system-wide caller wraps it in `sudo`. Both print where they wrote. Both land the same
+tree the `install()` rule does — the user-local copy applies that rule's file filter, and the
+prefix-wide target *is* that rule, reached through the `Help` component
+(`cmake --install --component Help`) — and the ordinary `cmake --install` still installs every
+component, so its behaviour is unchanged.
+
 Options: `--server`, `--port`, `--multicast`, `--interface`, `--buffer-mb` (per
 buffer, default 200), `--max-window`, `--holdoff`, `--recv-buffer`, `--plots` (the row
 count to *start* with, 1..`RowSet::kMaxRows`; the layout editor owns it afterwards),