Let a folder be starred, and put the starred ones on the home screen

The app bar of any folder now carries a star, and a starred folder appears in a Favourites section
on the home screen, above the libraries. Above, because it is the user's own list - and absent
entirely while empty, so someone opening the app for the first time still meets the libraries first,
which is the order they were given for a reason. The star in the row is the removal, which is what a
star means everywhere else: the mark that says "this is a favourite" is the thing you tap to stop it
being one. That avoids a long-press as well, and with it a destructive gesture nobody can see.

A favourite is a reference, not a copy: data/BookmarkStore keeps one path per row and nothing about
the folder's contents, so reading the whole list is one query and no filesystem work, which is what
makes it affordable on every visit to the home screen. It has its own SQLite database rather than a
table in the trash index - one table and three queries do not justify Room in a build that has no
annotation processor, and the two stores have nothing to say to each other. Two behaviours there are
decisions rather than defaults. A folder that has gone is kept and marked (Folder not found) instead
of being deleted: a card out of its slot and a folder deleted in another app are indistinguishable
from here, and silently forgetting someone's list because their SD card was out is the worse of the
two mistakes. And opening the row re-checks existence at that moment, because the flag is as old as
the screen - so the answer is a message rather than an empty listing, which would read as an empty
folder instead of a missing one.

Two smaller refusals came out of the same question. There is no star in a library or on the root
screen, because neither is a folder and there would be nothing to come back to: the distinction that
governs the new-folder button, and one the view model enforces too rather than trusting a stale
screen. And a bookmark has no undo, because Undo means "the last change to the filesystem the app
made" and a favourite is not one - the star is one tap to put back. The row's label comes from the
same place the breadcrumbs do, which is why a starred volume root reads as "Internal storage" rather
than as "0".

Sixteen tests: nine against real SQLite through Robolectric (round trip, ordering, starring twice
neither duplicating nor moving a row, and the same no-drop migration guard the trash index carries),
and seven through the view model (toggling, outliving the view model that added it, newest first,
the missing folder kept and marked, the missing folder refusing to open, and a library refusing to be
starred). 371 JVM tests, lint clean. Verified on the phone: the star appears only inside a folder and
flips from "Add to favourites" to "Remove from favourites" without leaving the screen; the home
screen lists the volume root above the libraries with its own label and its path underneath; the row
opens it with the star already filled; and the row's star takes the whole section away with a
snackbar. The documentation moved with it - CHANGELOG, ROADMAP (M15), README, COMPETITIVE_ANALYSIS
(that gap closes instead of joining the candidates), EDGE_CASE_TESTING, and the generated code map.

README.md

@@ -12,16 +12,19 @@ no advertising or tracking SDKs.
 
 ## Status
 
-**M14 — renaming a batch: done.** Selecting several files and choosing Rename opens a **pattern**
-dialog instead of the single-file one: one name, the number to start at, and a preview of the names
+**M15 — favourites: done.** A star in any folder's app bar puts that folder on the home screen, above
+the libraries, where a folder that has gone is marked *not found* rather than silently dropped and
+the row's own star removes it. Before it: **M14 — renaming a batch** (selecting several files and
+choosing Rename opens a **pattern** dialog instead of the single-file one: one name, the number to
+start at, and a preview of the names
 it will produce — and the numbering **carries on from the folder's own**, so `series (1)` and
 `series (2)` already there make the next batch start at 3. A batch that cannot be done is refused
 before anything moves, naming the file in the way, and the whole batch is one undo step. Before it:
 **M13 — video thumbnails** (a video row shows a frame of the video itself rather than a film icon —
 the platform's own retriever, through Coil, with a frame taken a second in and the icon kept as the
 fallback when a file cannot be decoded), **M12 — duplicate finder** (size, then the head of every
-candidate, then the survivors in full), **M11 — library views** (a **Libraries**
-section on the root screen — Images, Videos, Audio, Documents, Archives and Recent — whose rows open
+candidate, then the survivors in full), **M11 — library views** (a **Libraries** section on the
+root screen — Images, Videos, Audio, Documents, Archives and Recent — whose rows open
 ordinary listings of those files, each carrying a count from the search index), and **M10 — store
 assets**: the app is named **Plain Files**, the icon and feature graphic come from the app's own
 vectors, and [docs/STORE_LISTING.md](docs/STORE_LISTING.md) holds length-checked listing copy.