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.

docs/COMPETITIVE_ANALYSIS.md

@@ -308,7 +308,7 @@ requested feature in the market leader, and the permission line is a claim it ca
 | Automatic category libraries (Images / Audio / Videos / Documents / Downloads / New files / Recent) | **No** | The most-praised organisational feature in the reviews, and the one thing a brand-new user sees before they understand folders. We open on a volume list instead. This is the largest honest gap |
 | Duplicate finder | **No** | Explicitly requested, with an offer to pay. Pure local computation over a listing we can already do |
 | Batch rename, including continuing an existing numbering sequence | **No** | **Closed in M14.** Several files are renamed to one name with a number after each, and the numbering continues the folder's own series, so the "Ice (3), Ice (4)" wish from the top complainant is what the dialog now defaults to |
-| Bookmarks | **No** | Their action list includes it; we have none |
+| Bookmarks | **No** | **Closed in M15.** A star on any folder puts it in **Favourites** on the home screen, above the libraries; a folder that has gone is kept and marked rather than dropped |
 | "Copy again" / "Return to source folder" after a paste | **No** | Shipped in their 3.6.8 and cheap for us — the post-paste moment is when a user most needs a way back |
 | New-file dot after an operation | **No** | Their 3.6.5. Cheap, and it answers "what did that 4 GB copy just do?" |
 | Date-format setting | **No** | We deliberately leave the timestamp pattern unlocalised (README); a *user setting* is a different thing |
@@ -355,8 +355,9 @@ In the ROADMAP's own vocabulary (💡 = candidate, proposed, not scheduled; ✅
 2. ✅ **Duplicate finder** — shipped as **M12**. Asked for by name, by a user offering to pay.
 3. ✅ **Batch rename with sequence continuation** — shipped as **M14**, including the "continue from
    Ice (2)" case that the top complainant names.
-4. 💡 **Bookmarks, plus "Return to source folder" and "Copy again" after a paste.** Cheap, local,
-   and the post-paste moment is the one the competitor's changelog calls out.
+4. ✅ **Bookmarks** — shipped as **M15**. 💡 **"Return to source folder" and "Copy again"** after a
+   paste remain: cheap, local, and the post-paste moment is the one the competitor's changelog
+   calls out.
 5. 💡 **Highlight files changed by an operation.** Their 3.6.5; answers an obvious question.
 6. **Thumbnail hardening before the next release**: register an explicit Coil `ImageLoader` with a
    decided cache policy and a placeholder, and record a large-folder device pass in