feat(bookspec): GnuCash as the second engine, its differences settled (P13 step 5)

bookspec-gnucash implements the Engine trait over the oracle container
(oracle/gnucash/books.py) and claims the rules whose examples ask only
for balances. It passes BS-entry-3, BS-ledger-3, BS-statement-1 and
BS-statement-2 and differs from three examples, each adjudicated from the
sources and pinned by tests: GnuCash accepts an unbalanced entry and
parks the difference in an Imbalance account (BS-entry-1 refuses it); it
reads a credit of minus 100 as a debit (BS-entry-2 is a convention that
removes the ambiguity); it rounds 10.005 to 10.01 silently (BS-ledger-1
is the owner's choice not to). The example stands in each case: GnuCash
is a cross-check, not the authority.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

Cargo.toml

@@ -3,7 +3,7 @@
 # repository (P13 step 6, D-34).
 [workspace]
 resolver = "2"
-members = ["checker", "runner"]
+members = ["checker", "runner", "gnucash"]
 
 [workspace.package]
 edition = "2024"