docs: record that destinations are configuration

Beside the three navigation paragraphs, since it is the fourth thing
the shell states about where the user can go.

Two notes are measured facts rather than design: a default expressed as
an *absent key* survives an existing seed, where one expressed as a zero
value does not; and a spec can no longer assume a destination has a nav
item.

.planning/NOTES.md

@@ -3696,3 +3696,39 @@ as a tidy-up, a change nothing on a desktop renders differently.
 Related: a width-gated decision **is** testable at both tiers, which is
 why #61's phone mini player is a `matchMedia` stub in the component test
 and needs nothing special.
+
+## A default that is an *absent* key survives an existing seed (2026-08-19)
+
+The skill warns that a seed freezes every default it has already
+persisted, so changing one in `backend/config` is invisible against an
+existing `YJ_HOME` while CI, which seeds by running the app, tests the
+new one. That warning is about defaults stored as *values*.
+
+#25's Autotag-hidden default is stored as the **absence of a key**:
+`GeneralConfig.ViewVisibility` is a map, an id it does not mention takes
+`backend/config.Views`' answer, and only what the user changed is ever
+written. So a seed built before the feature existed showed the new
+default immediately — verified against `.dev/seeds/default.tar`, whose
+`config.toml` has no `[General.ViewVisibility]` table at all, and whose
+sidebar came up without Autotag on the first launch of the new binary.
+After toggling it on and off again the file carries exactly one line,
+`autotag = false`.
+
+The general form is worth keeping: **a default expressed as a zero value
+needs a re-seed to observe; a default expressed as an absent key does
+not**, and it needs no migration for existing installs either. It is the
+same property that makes removing a view later free (an unknown key is
+dropped on load), which is what the `#25 → #27` ordering on #73 rests on.
+
+## A spec cannot assume a destination has a nav item (2026-08-19)
+
+Since #25, `getByTestId('nav-<view>')` is not a reliable way to reach a
+view: Autotag is hidden by default and Downloads is absent without a
+download client, so four existing specs failed on a 30 s timeout waiting
+for a locator that will never resolve. `navigateTo(page, view)` in
+`e2e/support/fixtures.ts` dispatches the app's own `navigate` event
+instead, which is what every nav item, card and detail view dispatches —
+so it is the mechanism and not a test-only door.
+
+Use the nav item when the *nav* is the subject, and `navigateTo` when
+the view is.