Fix #926: the Libraries panel's search, groups and library export/import

Search: a search field at the top of the Libraries panel filters the
current library's colours, character and paragraph styles and graphics
by name, ignoring case; groups without a match are hidden, groups with
one open, and a search nothing matches says so ("No results").

Groups: items can be put in user-named groups, kept in the library file
(`groups: [{name, items: [{kind, item}]}]`, left out when there are
none, so files written before groups load as they are). New commands:
library.createGroup {library?, name?, items?}, library.renameGroup
{library?, group, name}, library.deleteGroup {library?, group} (its
items stay, ungrouped) and library.moveItem {library?, kind, item,
group?}. An item is in one group at most; deleting an item takes it out
of its group. The panel shows each group as a collapsible section with
its item count after the ungrouped items (rename and delete on its
context menu), each item's context menu has Move to Group (No Group,
the groups, New Group...), and a New Group button sits in the footer.
library.get lists the groups, library.list counts them. The generic
command dialog takes fixed params in `__params` for this.

Export / import: library.export {library?, path?} writes a library
(colours, styles, graphics with art and thumbnails, groups) as a
.vclibrary file, the JSON the library folder keeps (without a path:
{data}); library.import {path | data | dataBase64, name?} adds one as a
new library, made current, numbered on a name clash ("Brand (2)").
The panel menu has Import Library... and Export Library..., through the
app's file dialogs (the web downloads, and picks through File > Open,
which now lists Libraries files). Imported and folder files are made
safe: at most 10,000 items of a kind and 1,000 groups, names clipped and
made unique, sizes finite, thumbnails kept only when their PNG header
says at most 1024 px a side, group entries naming missing items or
unknown kinds dropped; a file over 512 MB or that isn't a library is an
error. Paths go through the engine's file helpers (automation roots).
The importer is fuzzed (import_fuzz: garbage and mutated libraries).

Also fixed: the third graphic added to a library got the second's id
(`graphic-2` twice).

Docs: the commands' params docs, docs/mcp.md, ROADMAP.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/mcp.md

@@ -953,9 +953,26 @@ gives another colour merges into the document's), a colour paints the selection'
 to the document (numbered when its name is taken by other attributes) and applied to the selected text, each as one
 undo step. `library.removeItem {library?, kind, item}` removes one.
 
+Items can be put in user-named groups, which the panel shows as collapsible sections after the ungrouped items (its
+search field filters every item by name; `library.get` lists them all to filter). `library.createGroup {library?,
+name?, items?: [{kind, item}]}` makes one (a name another group has, ignoring case, gets a number) with those items
+moved into it; `library.renameGroup {library?, group, name}` renames one; `library.deleteGroup {library?, group}`
+deletes one, its items staying in the library ungrouped; and `library.moveItem {library?, kind, item, group?}` moves
+an item into a group (without `group`, out of its group). An item is in one group at most; `library.get` lists the
+groups as `groups: [{name, items: [{kind: "color"|"charStyle"|"paraStyle"|"graphic", item}]}]` (a graphic by its id).
+
+`library.export {library?, path?}` writes a library (its colours, styles, graphics with their art and thumbnails, and
+its groups) as a `.vclibrary` file, the JSON the library folder keeps (without `path`: `{data}`, the file's text).
+`library.import {path? | data? | dataBase64?, name?}` adds the library of such a file as a new library and makes it
+the current one; a name another library has gets a number (`Brand (2)`). Imported files are checked: at most 10,000
+items of each kind and 1,000 groups, names clipped and made unique, thumbnails kept only when they are small PNGs, and
+groups naming items that aren't there dropped. Files written before groups load as they are.
+
 ```json
 {"name":"run_command","arguments":{"command":"library.add","params":{"kind":"graphic","name":"Logo"}}}
 {"name":"run_command","arguments":{"command":"library.use","params":{"kind":"graphic","item":"Logo","center":[300,200]}}}
+{"name":"run_command","arguments":{"command":"library.createGroup","params":{"name":"Logos","items":[{"kind":"graphic","item":"Logo"}]}}}
+{"name":"run_command","arguments":{"command":"library.export","params":{"path":"/tmp/Brand.vclibrary"}}}
 ```
 
 ## Strokes on type