feat: general ledger by account, trial balance as of, members' equity

Three reports a CPA expects that the engine could not produce:

- general_ledger(start, end): the canonical general ledger, organized by
  account in code order. Each account has its beginning balance, every
  posting in the period (date, entry, description, debit, credit, running
  balance on the normal side) and its ending balance, which ties to the
  trial balance. Each posting names its split: the one account on the
  opposite side of its entry, or "-Split-" when that side has several,
  so a compound entry (one debit, several credits) reads correctly from
  every account it touches. The entry-ordered listing the exports called a
  "general ledger" is really the general journal.
- trial_balance_as_of(as_of): a trial balance is always as of a date.
- members_equity(start, end, members): the statement of changes in
  members' equity for a multi-member LLC taxed as a partnership. Each
  member's capital roll-forward: beginning (capital plus their share of
  undistributed earnings), contributions, allocated net income (loss),
  distributions, ending. Allocation is by ownership share to the cent,
  the last member absorbing rounding, so totals are exact and the ending
  total equals balance-sheet equity; any equity change the roll-forward
  can't explain shows as "other" instead of disappearing. Shares must be
  positive and sum to exactly 1; capital accounts must be distinct
  equity accounts (LedgerError::InvalidMembers).
- CashFlowLine.account: the account a line is about, so renderers can
  name members instead of account codes.

Tests (every expectation computed by hand): trial balance as of Jan 31;
February general ledger with beginning balances, running balances, a
card that nets to zero, an inactive account still listed, and every
ending balance tied to the trial balance; a compound entry's splits from
all three sides; a two-member partnership where 666.67 and 100.01 split
unevenly (333.34/333.33, 50.01/50.00), with and without closing January,
tying to the balance sheet each month; bad member configuration.

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

crates/ledger-core/src/error.rs

@@ -132,6 +132,10 @@ pub enum LedgerError {
     #[error("purchase date plus useful life overflows the calendar")]
     AssetDateOverflow,
 
+    /// Partnership members are misconfigured (shares, capital accounts).
+    #[error("invalid members: {0}")]
+    InvalidMembers(String),
+
     /// Cash-flow period end is before period start.
     #[error("invalid date range: end {end} is before start {start}")]
     InvalidDateRange {