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]>

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