refactor(bookspec): the specification lives in its own repository (P13 step 6)

bookspec/ (spec, examples, checker, runner, Lean, GnuCash adapter) moves to
https://git.swaits.com/swaits/bookspec, history kept by git subtree split
(public home; CC BY 4.0 for spec/ and examples/, MIT OR Apache-2.0 for the
code). ledger-core and ledger-package depend on bookspec-checker and
bookspec-runner as git dev-dependencies pinned to
74f2853d5f0db0066147c17fed8033cfe3d83a24, and read the examples through
bookspec_checker::examples(). just check still runs every example via
cargo test --workspace. The pin check against docs/spec/sources and the
rules.md id check move into tests/bookspec.rs (they need this repository's
files). The accounting is unchanged: no example or rule moved.

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

README.md

@@ -128,7 +128,7 @@ crates/ledger-package/ # the shared read-only view: the CPA package, the invoice
 crates/ledger-vault/   # the TIN vault (XChaCha20-Poly1305 under the book's key)
 crates/ledger-gui/     # the viewer: read-only, live (egui)
 verify/lean/           # the accounting laws as Lean theorems (`just prove`)
-bookspec/              # the formal accounting specification: checker, runner, examples (P13)
+# the formal accounting specification lives in its own repository, https://git.swaits.com/swaits/bookspec (pinned dev-dependency, P13)
 fuzz/, oracle/         # fuzz targets; the GnuCash differential (`just oracle`)
 deploy/                # the self-host recipe and system unit (P9a)
 site/                  # wanderbooks.ai and install.sh