P3: rename accounts, one set of period defaults, tax categories warn or refuse (defects 15, 16, 8)

books.rename_account; account names trimmed, bounded, refused when a
sibling's or TIN-shaped; books.show refuses what its target ignores. A
report asked for without dates covers this fiscal year so far, everywhere
(one rule for tools, descriptions and the viewer; the IIF and CPA package
keep their own dates). policy.require_tax_category warns or refuses; a
Form 4797 gain is shown apart on every return.

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

crates/ledger-core/src/lib.rs

@@ -92,4 +92,6 @@ pub use schedules::{
     ScheduleId, ScheduleKind, TemplateLine,
 };
 pub use shares::{PartyMember, PartyShare, Share};
-pub use tax::{TaxCategory, TaxForm, TaxKind, TaxSummary, TaxSummaryLine, tax_summary};
+pub use tax::{
+    BookOnlyLine, TaxCategory, TaxForm, TaxKind, TaxSummary, TaxSummaryLine, tax_summary,
+};