search: a write reads only its folder, and the match is lit

`file_text::catch_up` read every file without a row, which after a write meant
walking every file node there is to find the one or two just put in. It now
takes the part of the tree to look in: an upload asks about the folder it went
into, an import about the folder it was poured into, an SFTP save about the
folder it landed in, a decrypt about what was decrypted. Only an import at the
root and the pass at start look at everything.

The excerpt of a file found by its text marks each matched word between
U+0002 and U+0003, which no text file has a use for, and the list draws them
lit. A cut through a match leaves an opening without its close, and the rest
is lit to the end rather than the marker shown.

The screenshots are taken again, so the search one shows it.

docs/wiki/Files-and-Search.md

@@ -68,7 +68,7 @@ A search reads the text files in the tree as well as the documents. A file
 counts as text by its type, or when its first 8 KiB read as UTF-8 with no NUL
 byte, which covers `.log` files and Makefiles. The first 256 KiB of each is
 indexed, the same part the viewer shows. A file found by its text shows the
-words around the match under its name.
+words around the match under its name, with the words that matched lit.
 
 The text is read when a file arrives, by upload, import or the SFTP mount,
 again when its bytes change, and at start for any file still missing it. PDFs,