feat: tax categories and the tax summary report

wanderbooks is a general MCP-first accounting application: every
business needs its accounts mapped to where they land on its income tax
return, and its CPA needs book-to-tax differences surfaced, not hidden.

- ledger_core::tax: a catalog of stable categories (gross_receipts,
  commissions_fees, meals, guaranteed_payments, officer_compensation,
  nondeductible, ...) with kind (income, cost of goods, deduction), the
  returns each appears on (Schedule C, 1065, 1120-S, 1120), a default
  deductible percent (meals 50, nondeductible 0), and guidance. Keys,
  not line numbers: form lines move from year to year.
- Accounts carry tax_category and an optional deductible_percent
  override, validated against the account type (and the return, at the
  MCP layer), journaled as account.tax_set, replayed, projected (schema
  v6).
- tax_summary(start, end, form): the P&L grouped by category, book and
  deductible amounts (half away from zero to the cent), contra income
  (refunds) reducing receipts, and every account still uncategorized.
- MCP: books.tax_categories (filtered to the configured return),
  books.set_tax_category, a tax_summary report in every format and in
  the CPA package, chart output with categories, and a fifth
  tax_category column in chart CSV imports, validated before anything is
  created. [company] tax_form, or derived from tax_classification.

Scope stays book basis: no tax depreciation, basis, or filing.

Tests: a hand-computed quarter (50% meals 333.33 -> 166.67, penalties at
0%, refunds as negative receipts, guaranteed payments, one uncategorized
account; book net 5,276.67 vs tax 5,483.33); overrides; category/account
/return validation; form parsing and derivation; CSV parsing; stdio:
filtered catalog, mapping, refusal, summary, all-or-nothing import.

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

crates/ledger-core/src/account.rs

@@ -6,6 +6,7 @@
 //! grouping on type, so getting this right is the whole game.
 
 use crate::{Currency, LedgerError};
+use rust_decimal::Decimal;
 use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
 
@@ -72,6 +73,14 @@ pub struct Account {
     pub placeholder: bool,
     /// Day one: always the ledger's base currency (see R-5).
     pub currency: Currency,
+    /// Tax category key (see [`crate::tax`]), for revenue and expense
+    /// accounts.
+    #[serde(default)]
+    pub tax_category: Option<String>,
+    /// Percent deductible, overriding the category's default (e.g. 50 for
+    /// meals, 0 for penalties).
+    #[serde(default)]
+    pub deductible_percent: Option<Decimal>,
 }
 
 /// The registry of accounts. Owns id allocation for accounts it holds.
@@ -116,6 +125,22 @@ impl ChartOfAccounts {
         self.accounts.values()
     }
 
+    /// Set (or clear) an account's tax category and deductible percent.
+    pub fn set_tax(
+        &mut self,
+        id: AccountId,
+        category: Option<String>,
+        deductible_percent: Option<Decimal>,
+    ) -> Result<(), LedgerError> {
+        let a = self
+            .accounts
+            .get_mut(&id)
+            .ok_or(LedgerError::UnknownAccount(id))?;
+        a.tax_category = category;
+        a.deductible_percent = deductible_percent;
+        Ok(())
+    }
+
     /// Number of accounts in the chart.
     pub fn len(&self) -> usize {
         self.accounts.len()