fix(core,store): child accounts share their parent's type; periods nest; lines checked when added

Three spec decisions from Stephen (2026-10-13), each test-first:

- A child account has its parent's type (ParentTypeMismatch): a child
  subdivides its parent. Test: a_child_account_has_its_parents_type.
- Periods nest (a month in a quarter in a year) but never partially
  overlap (PeriodsOverlap; a refusal uses no id). Test:
  periods_nest_but_never_partially_overlap.
- A line's account must exist and take postings when the line is added,
  not only at post (UnknownAccount / PlaceholderAccount from add_line
  and add_line_fx); posting still re-checks. Two older tests that pinned
  the post-time refusal now check both (the post-time check is reached
  through the unchecked replay path, as old journals can).

Compatibility: replay reads history through new Ledger::replay_*
commands (add_account, add_line, add_line_fx, create_period) that skip
checks added later, so books written before these rules still open.
The contract test books_written_under_older_rules_still_open writes such
a journal directly and found, on its first run, that the SQLite
cross-check rebuild used the checked commands and would have failed
verification ("sqlite projection can't be read back"); it now replays
too. Documented in docs/spec/README.md; decisions recorded in rules.md
(B-1, B-4, B-12).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

crates/ledger-core/src/error.rs

@@ -123,6 +123,42 @@ pub enum LedgerError {
         spec: String,
     },
 
+    /// A child account subdivides its parent, so it must be the same type
+    /// (Stephen, 2026-10-13).
+    #[error(
+        "account {code} is {found:?}, but its parent {parent_code} is {expected:?}; a child account must be the same type as its parent"
+    )]
+    ParentTypeMismatch {
+        /// The new account's code.
+        code: String,
+        /// The parent's code.
+        parent_code: String,
+        /// The parent's type, which the child must share.
+        expected: crate::AccountType,
+        /// The type the child was given.
+        found: crate::AccountType,
+    },
+
+    /// Periods nest (a month in a quarter in a year) but never partially
+    /// overlap (Stephen, 2026-10-13).
+    #[error(
+        "period {name} ({start} to {end}) partially overlaps period {other} ({other_start} to {other_end}); periods may nest, but not overlap"
+    )]
+    PeriodsOverlap {
+        /// The new period's name.
+        name: String,
+        /// Its first day.
+        start: chrono::NaiveDate,
+        /// Its last day.
+        end: chrono::NaiveDate,
+        /// The existing period's name.
+        other: String,
+        /// The existing period's first day.
+        other_start: chrono::NaiveDate,
+        /// The existing period's last day.
+        other_end: chrono::NaiveDate,
+    },
+
     /// The referenced journal entry does not exist.
     #[error("entry {0} does not exist")]
     UnknownEntry(u64),