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/ledger.rs

@@ -228,10 +228,55 @@ impl Ledger {
             parent,
             placeholder,
             currency: self.base_currency,
+            tax_category: None,
+            deductible_percent: None,
         })?;
         Ok(id)
     }
 
+    /// Set (or clear, with `None`) an account's tax category, and optionally
+    /// override the category's deductible percent (0 to 100). The category
+    /// must fit the account's type (see [`crate::tax::check_category`]).
+    pub fn set_account_tax(
+        &mut self,
+        account: AccountId,
+        category: Option<&str>,
+        deductible_percent: Option<Decimal>,
+    ) -> Result<(), LedgerError> {
+        self.check_account_tax(account, category, deductible_percent)?;
+        self.chart
+            .set_tax(account, category.map(str::to_string), deductible_percent)
+    }
+
+    /// The validation behind [`Self::set_account_tax`], for stores that
+    /// journal before mutating.
+    pub fn check_account_tax(
+        &self,
+        account: AccountId,
+        category: Option<&str>,
+        deductible_percent: Option<Decimal>,
+    ) -> Result<(), LedgerError> {
+        let a = self
+            .chart
+            .get(account)
+            .ok_or(LedgerError::UnknownAccount(account))?;
+        if let Some(key) = category {
+            crate::tax::check_category(key, a.account_type, None)?;
+        } else if deductible_percent.is_some() {
+            return Err(LedgerError::InvalidTaxCategory(
+                "a deductible percent needs a tax category".to_string(),
+            ));
+        }
+        if let Some(p) = deductible_percent
+            && (p < Decimal::ZERO || p > Decimal::from(100))
+        {
+            return Err(LedgerError::InvalidTaxCategory(format!(
+                "deductible percent {p} must be between 0 and 100"
+            )));
+        }
+        Ok(())
+    }
+
     /// Start a draft journal entry. Returns its id.
     ///
     /// This is the unchecked constructor: replay and internal callers