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-mcp/tests/mcp_stdio.rs

@@ -688,7 +688,7 @@ fn mcp_resources_over_stdio() {
     );
     assert_eq!(init["result"]["serverInfo"]["name"], "wanderbooks");
     // Resources are advertised.
-    assert_eq!(init["result"]["capabilities"]["resources"].is_null(), false);
+    assert!(!init["result"]["capabilities"]["resources"].is_null());
     mcp.notify("notifications/initialized");
 
     mcp.call(