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.

docs/EDGE_CASE_TESTING.md

@@ -192,6 +192,46 @@ through the prompt, because the prompt was already covered in the M7 session.
   the AVDs have no `/dev/kvm` here, and the two-pane root list is where the libraries now appear
   first.
 
+## The batch rename session (M14)
+
+Driven on the Xiaomi phone against a purpose-made folder — `/sdcard/AAABatchRename`, holding
+`alpha.txt`, `beta.txt`, `gamma.txt` and an existing series `series (1).txt` / `series (2).txt` —
+because the one thing worth proving here is the **continuation**, and that needs a folder that
+already holds a series. Every step went through the UI with `adb input`, and each claim below was
+read back off the device rather than off the screen.
+
+- **The dialog continues the series on its own.** Three files selected (long-press, then a tap each —
+  the M4 tap-to-toggles behaviour), Rename from the action bar, `series` typed into the name and the
+  **Start at** field moved from 1 to **3** by itself, with the preview reading *Will be renamed to
+  series (3).txt, series (4).txt, series (5).txt*. Nothing but the names in the folder was consulted,
+  and those are the listing on screen, so the dialog and the view model cannot disagree about them.
+- **The work is exactly what the dialog promised.** `ls` afterwards showed `series (1) … (5)`, the
+  two originals untouched, and `cat` showed `series (3).txt` holding `alpha` and `series (4).txt`
+  holding `beta` — the renames *moved* the files rather than copying them.
+- **One undo step, through the snackbar.** A second batch was renamed to `clip (1..3)`, the snackbar
+  read *Renamed 3 files* with **Undo**, and a single tap put all three names back, contents intact
+  through the round trip (`series (5).txt` still held `gamma`).
+- **Both refusals are visible before anything can be pressed.** With `series` typed and the start
+  forced back to 1 over a folder holding `series (1)`/`series (2)`, the dialog read *series (1).txt is
+  already in this folder* — the file in the way, named — and the only `enabled="false"` node in the
+  whole dump was the Rename button. Renaming two files to the names they already hold read *Every
+  selected file already has that name* instead, which is the honest version of "nothing to do", and a
+  name containing `/` read *A name cannot contain a slash*, button disabled as well.
+- **The test folder was removed afterwards.** It was created for this pass, in the phone's own
+  storage, and deleted through `adb` at the end: the M10 rule about not leaving artefacts on a device
+  that is not ours applies to a scratch folder as much as to screenshots.
+
+Two things this session added to the tooling notes:
+
+- **`adb input text` does not deliver a `/` reliably.** `'a/b'` arrived in the field mangled (`a!c`,
+  after the trailing character had to be deleted), so the slash case was sent as `input keyevent 76`
+  — worth knowing before concluding that a field filters separators: here the *field* accepted it and
+  the dialog is what refused it.
+- **The dialog lifts once the keyboard appears.** Its fields sit ~200 px higher afterwards, so a
+  second field has to be aimed at the post-keyboard layout. Two dumps were needed to tell the two
+  positions apart, which is also why the numbers quoted above come from a dump rather than from
+  memory.
+
 ## Not verified, and why
 
 - **Long-press multi-select via adb: it works, but not every time.** `input swipe x y x y 900`