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

docs/requirements.md

@@ -65,6 +65,14 @@ requirement conflict, the requirement wins.
   to the cent, the last member absorbing rounding, so every row's total is
   exact. Any equity change the roll-forward doesn't explain is shown as
   "other", never hidden.
+- **R-9d — Tax categories.** Revenue and expense accounts map to stable
+  tax categories (`ledger_core::tax::CATEGORIES`: key, label, income /
+  cost of goods / deduction, the returns it appears on — Schedule C, 1065,
+  1120-S, 1120 — default deductible percent, guidance), with an optional
+  per-account deductible percent. `tax_summary(start, end, form)` groups
+  the P&L by category with book and deductible amounts and lists
+  uncategorized accounts. Book basis only: no tax depreciation, basis, or
+  filing; not tax advice.
 - **R-10 — Canonical snapshots.** `reports::snapshot()` renders the three
   core reports as deterministic JSON (decimal strings, sorted lines, fixed
   keys). Two engines fed the same books must produce identical output. This