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/COMPETITIVE_ANALYSIS.md

@@ -307,7 +307,7 @@ 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** | We have single-item rename only; "Ice (3), Ice (4)" is a stated wish from the top complainant |
+| 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 |
 | "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?" |
@@ -346,13 +346,15 @@ latency, so this needs to be pinned down rather than assumed:
 
 ## What I would do with this
 
-In the ROADMAP's own vocabulary (💡 = candidate, proposed, not scheduled):
+In the ROADMAP's own vocabulary (💡 = candidate, proposed, not scheduled; ✅ = since shipped):
 
-1. 💡 **Category libraries (Images / Audio / Videos / Documents / Downloads · New files · Recent).**
-   The largest honest gap: it is the market leader's most-praised organisation feature, it needs no
-   permission, and it is the first thing a new user sees.
-2. 💡 **Duplicate finder.** Asked for by name, by a user offering to pay.
-3. 💡 **Batch rename with sequence continuation** — including the "continue from Ice (2)" case.
+1. ✅ **Category libraries (Images / Audio / Videos / Documents · New files · Recent)** — shipped as
+   **M11**, with Downloads and New files left out on purpose. It was the largest honest gap: the
+   market leader's most-praised organisation feature, needing no permission, and the first thing a
+   new user sees.
+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.
 5. 💡 **Highlight files changed by an operation.** Their 3.6.5; answers an obvious question.