chore!: the converter is named papervert (short form pvt)

paperstract becomes papervert, the paper converter: package, command,
PAPERVERT_* variables, the [papervert] settings section, tools/pvt.py
and the .pvt- CSS classes. No old name is kept as an alias.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017R7sgDNvUGWY2zFPCq2JH6

CHANGELOG.md

@@ -38,7 +38,7 @@ Packaging, commands and lazy imports only; the formats stay at 0.1.0, and no com
 
 ### Unreleased
 
-Two changes asked for by the first producer that compares a converted reading with a second witness (paperstract with a JATS file).
+Two changes asked for by the first producer that compares a converted reading with a second witness (papervert with a JATS file).
 Both are minor changes in the sense of [spec/00](spec/00-introduction.md): no valid file becomes invalid, and no hash, ID or key changes; the next release numbers them.
 
 - Alignment has a substitution step ([spec/05 §5.6](spec/05-hashes.md#56-alignment), step 3): in each stretch between pairs, an unpaired sentence of the first reading is paired with the first unpaired sentence of the second whose similarity (2*L* / (|*a*| + |*b*|) over code points of the lossy projections, *L* their longest common subsequence) is at least 0.8, both of prose length 20 or more.