P3 review: P2 can read the books until the owner consents

A schedule-convention setting, the two new tax categories, a disposal
before the in-service date and a depreciation catch-up each need their
feature now (tax_categories_gain_exempt, disposal_before_service,
depreciation_catch_up), so no write before consent leaves books P2 refuses
or misreads; replayed names are read with P2's TIN detector (B-28). The
cutover runs end to end through the real P2 and P3 binaries in a test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

crates/ledger-mcp/src/server.rs

@@ -6017,6 +6017,14 @@ impl BooksServer {
                     self.settings().company.tax_form,
                 )
                 .map_err(|e| format!("{}: {e}", row.code))?;
+                // A category an earlier version doesn't know waits for the
+                // books' format upgrade: refused here, with the file whole,
+                // never after the rows before it were written.
+                if let Some(f) = ledger_store::format::tax_category_feature(cat) {
+                    store
+                        .ensure_feature(f)
+                        .map_err(|e| format!("{}: {e}", row.code))?;
+                }
                 scratch
                     .set_account_tax(id, Some(cat), None)
                     .map_err(|e| format!("{}: {e}", row.code))?;