chore: clippy clean across the workspace, all targets

Zero warnings from `cargo clippy --workspace --all-targets`, which is now
a hard rule rather than "clean except pre-existing":

- ChartOfAccounts::add takes the Account it registers instead of eight
  loose parameters (too_many_arguments); Ledger::add_account builds it.
  Same validation, same errors.
- close_period tool: collapse the nested if-let into a let chain.
- iif.rs docs: the IIF layout example shows tabs as `\t` instead of
  literal tab characters (tabs_in_doc_comments).
- Tests: drop an unused binding; assert! instead of assert_eq! against a
  literal bool.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MtYEaR6M6Ybdhm6X98BWNu

crates/ledger-import/src/iif.rs

@@ -1,14 +1,14 @@
 //! Generate Intuit Interchange Format (IIF) journal entries for QuickBooks
 //! Desktop, which many CPAs still run.
 //!
-//! Layout per entry:
+//! Layout per entry (fields are tab-separated; `\t` marks each tab):
 //!
 //! ```text
-//! !TRNS	TRNSTYPE	DATE	ACCNT	NAME	AMOUNT	DOCNUM	MEMO
-//! !SPL	TRNSTYPE	DATE	ACCNT	AMOUNT	DOCNUM	MEMO
+//! !TRNS\tTRNSTYPE\tDATE\tACCNT\tNAME\tAMOUNT\tDOCNUM\tMEMO
+//! !SPL\tTRNSTYPE\tDATE\tACCNT\tAMOUNT\tDOCNUM\tMEMO
 //! !ENDTRNS
-//! TRNS	GENERAL JOURNAL	01/15/2026	Cash		1000.00	1	Memo
-//! SPL	GENERAL JOURNAL	01/15/2026	Revenue	-1000.00	1	Memo
+//! TRNS\tGENERAL JOURNAL\t01/15/2026\tCash\t\t1000.00\t1\tMemo
+//! SPL\tGENERAL JOURNAL\t01/15/2026\tRevenue\t-1000.00\t1\tMemo
 //! ENDTRNS
 //! ```
 //!