Rename a whole batch at once, numbered on from the folder's own series

Selecting more than one file and choosing Rename now opens a pattern dialog rather than the
single-file one: a name, the number to start at, and a live preview of the names it will produce.
The number is the part that makes the feature worth having. A batch rename that always starts at 1
is useless the second time it is used, because the first batch already owns 1 to 12, so the dialog
reads the names already in the folder and starts at the highest "base (n)" it finds: a folder
holding "series (1)" and "series (2)" makes the next batch start at 3, which is the market survey's
most-helpful complaint answered exactly ("if I later find another group of photos which I want to
continue the batch naming with 'Ice (3)...'"). Three rules in core/BatchRename are decisions rather
than defaults, and are written down as such: the extension is kept per file, so a mixed batch of
.jpg and .png keeps each file's own; the series is counted across extensions, because "Holiday
(9).png" making the next number 10 is one series while "Holiday (10).jpg" beside "Holiday (1).png"
is two; and it continues from the highest number rather than from the first gap, because someone
renaming a second batch is carrying a series on rather than filling in the holes of one, with the
start number theirs to type where a gap matters more.

The batch is planned in full before a single file moves, and the plan has three answers and
deliberately no fourth: Ready, Blocked (naming the target that is already in the folder) or
NothingToDo. A batch that renamed nine files of twelve and then stopped would leave the user to work
out which nine and to guess whether the last three crashed or were skipped, so a collision refuses
the whole batch, the dialog names the file in the way and disables its button as the name is typed,
and a chain is declined outright - "a.txt" taking "a (1).txt" while "a (1).txt" is itself in the
batch would need the files shuffled through temporary names. Every rename still goes through the
same FileOperations.rename a single rename uses, so the refusal to overwrite anything is the same
one, and the successful renames are collected into the single Undo.Relocate step that already
existed: a feature that touches twelve files at once owes the user one tap back. The action bar now
offers Rename to any selection, since which of the two dialogs a tap means is described by the
dialog that opens rather than by the bar; Details stays single, because "the details" of twelve
files is not a screen.

Writing it turned up a quieter bug in the rename that was already there. rename(path,
"notes/final.txt") built a child path, which resolves into the subfolder rather than alongside the
file, so a name containing a separator moved the file instead of renaming it and nothing in the
dialog or the listing showed that anything had happened. Both rename paths now refuse a name that is
not one path segment (core/FileNames.isUsableName), and the dialog says which rule it is while it is
typed, which is where the user can still do something about it.

Twenty new core tests (BatchRenameTest, FileNamesTest) and six through the view model, 350 JVM
tests in total, lint clean. Verified by driving it on the phone against a folder that already held
"series (1).txt" and "series (2).txt": with three files selected and "series" typed in, Start at
moved to 3 on its own and the preview listed "series (3).txt, series (4).txt, series (5).txt";
afterwards the folder held all five series files, each file's contents having travelled with its
rename and the two originals untouched, and the snackbar's Undo put a later batch of three back in
one step. The collision and the slash were driven the same way - the file in the way named with the
button disabled, and "A name cannot contain a slash" - and the scratch folder was removed from the
device afterwards. CHANGELOG, ROADMAP (M14), COMPETITIVE_ANALYSIS (the gap moves from candidate to
closed), STORE_LISTING and EDGE_CASE_TESTING carry the feature, its decisions and the pass itself;
two tooling notes went into the last of them, that "adb input text" does not deliver a slash and
that the dialog lifts once the keyboard appears.

README.md

@@ -12,10 +12,15 @@ no advertising or tracking SDKs.
 
 ## Status
 
-**M13 — video thumbnails: done.** 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. Before it: **M12 — duplicate finder** (size, then the
-head of every candidate, then the survivors in full), **M11 — library views** (a **Libraries**
+**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
+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
 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
@@ -174,6 +179,11 @@ UI.** Translated UI text has to be supplied in `values-<locale>/`.
   no archive encryption or password support.
 - **Cancellation happens between entries**, so a single very large file finishes before a
   stop is honoured.
+- **A batch rename is refused rather than half-done when one of its names is taken**, including when
+  the file holding that name is part of the same batch: `a.txt` taking `a (1).txt` while `a (1).txt`
+  is itself being renamed needs the files shuffled through temporary names, so the dialog names the
+  file in the way instead. Renaming in the other order, or starting the numbering past the block,
+  does what you want.
 - **A library is not restored across process death.** The persisted view state holds a path, and a
   library has none, so a restart returns to the root screen rather than to Images or Recent.
 - **Long-press multi-select is awkward to script from the host.** The gesture that opens the