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

docs/spec/README.md

@@ -31,6 +31,14 @@ SOURCE: CONVENTION (QuickBooks aging); nearest authority ASC 326-20-55-38 (illus
 A rule whose only source is CONVENTION or DECISION is a candidate for a
 human to confirm or overturn; nothing else is.
 
+## Rules added later never lock out older books
+
+When a rule is tightened (a check added to a command), books written
+before it must still open. Journal replay and the SQLite cross-check read
+history through the engine's `replay_*` commands, which skip checks added
+later; new commands go through the checked ones. Pinned by
+`books_written_under_older_rules_still_open` (ledger-store).
+
 ## Vendoring rules
 
 - Public-domain texts are copied **verbatim** (whitespace normalized), each