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, 3 insertions, 0 deletions

diff --git a/src/fs.zig b/src/fs.zig
index 1d0bbc22..e66a6a49 100644
--- a/src/fs.zig
+++ b/src/fs.zig
@@ -1335,6 +1335,9 @@ pub const Namespace = struct {
     /// 0 the workspace's, a column's its index + 1; and whether the newline
     /// that ended its last write is held back.
     header_rewrite: ?u32 = null,
+    /// A column's or the workspace's tag truncated, not yet done (as a
+    /// pane tag's `tag_trunc_pending`): its key, the workspace's 0.
+    header_trunc_pending: ?u32 = null,
     /// A look or exec a 9P write made found every pane slot taken; the
     /// write fails with what was said, kept at the head of `ename`.
     no_pane_slot: bool = false,