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

crates/ledger-core/Cargo.toml

@@ -26,8 +26,10 @@ proptest = "1"
 # store (a dev-dependency only: the store depends on this crate), each in
 # a directory of its own.
 toml = "1"
-bookspec-checker = { path = "../../bookspec/checker" }
-bookspec-runner = { path = "../../bookspec/runner" }
+# The specification lives in its own repository (P13 step 6, D-34, D-65),
+# pinned to a commit.
+bookspec-checker = { git = "https://git.swaits.com/swaits/bookspec", rev = "74f2853d5f0db0066147c17fed8033cfe3d83a24" }
+bookspec-runner = { git = "https://git.swaits.com/swaits/bookspec", rev = "74f2853d5f0db0066147c17fed8033cfe3d83a24" }
 ledger-package = { path = "../ledger-package" }
 ledger-import = { path = "../ledger-import" }
 ledger-store = { path = "../ledger-store" }