Pane, column and workspace tags take one set of write checks, and a refused `>` write leaves the tag as it was

A column's or the workspace's tag took any length, so a write past 4096 bytes went in and a later Dump failed on it (bad dump tag), and a truncating open wiped any tag before the write after it could be refused. The column and workspace tags now refuse what a pane tag refuses: the 4096-byte limit (ENOSPC, tag: no space: over 4096 bytes) as well as the control characters they already did. A truncation of any of the three is held until the write after it is known to fit, and done with it; a refused write drops it; a close or read with no write after it does it then (so `: > tag` still clears). A test runs a truncating write too long, a control character and a bare truncation at each kind, and Dumps after. docs/fs.md says so.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 4 insertions, 1 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 06fa1406..b30ae3ef 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -320,7 +320,10 @@ blank matters; `echo` would start a second line). A pane tag may hold
 several lines; a column or workspace tag is one, a newline written into it
 becoming a space. Control characters other than tab, DEL, C1 controls and
 non-UTF-8 bytes are refused (`invalid tag text`). The editable text is at
-most 4096 bytes (`tag: no space: over 4096 bytes`, ENOSPC). A clear is an ordinary edit
+most 4096 bytes (`tag: no space: over 4096 bytes`, ENOSPC). All three kinds
+take the same checks, whole or not at all, and a `>` whose write is refused
+changes nothing: its truncation is done with the write that fits, or at the
+close (or a read) when none came. A clear is an ordinary edit
 and `u` in the tag undoes it. [tags.md](tags.md) covers tags on screen.
 
 ## Panes