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

Cargo.lock

@@ -763,8 +763,10 @@ dependencies = [
 [[package]]
 name = "bookspec-checker"
 version = "0.1.0"
+source = "git+https://git.swaits.com/swaits/bookspec?rev=74f2853d5f0db0066147c17fed8033cfe3d83a24#74f2853d5f0db0066147c17fed8033cfe3d83a24"
 dependencies = [
  "chrono",
+ "include_dir",
  "rust_decimal",
  "serde",
  "serde_json",
@@ -774,6 +776,7 @@ dependencies = [
 [[package]]
 name = "bookspec-runner"
 version = "0.1.0"
+source = "git+https://git.swaits.com/swaits/bookspec?rev=74f2853d5f0db0066147c17fed8033cfe3d83a24#74f2853d5f0db0066147c17fed8033cfe3d83a24"
 dependencies = [
  "bookspec-checker",
  "serde_json",
@@ -3451,6 +3454,25 @@ version = "0.14.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "09e54e57b4c48b40f7aec75635392b12b3421fa26fe8b4332e63138ed278459c"
 
+[[package]]
+name = "include_dir"
+version = "0.7.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "923d117408f1e49d914f1a379a309cffe4f18c05cf4e3d12e613a15fc81bd0dd"
+dependencies = [
+ "include_dir_macros",
+]
+
+[[package]]
+name = "include_dir_macros"
+version = "0.7.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7cab85a7ed0bd5f0e76d93846e0147172bed2e2d3f859bcc33a8d9699cad1a75"
+dependencies = [
+ "proc-macro2",
+ "quote",
+]
+
 [[package]]
 name = "indexmap"
 version = "2.14.2"
@@ -4039,6 +4061,7 @@ dependencies = [
 name = "ledger-package"
 version = "0.1.0"
 dependencies = [
+ "bookspec-checker",
  "chrono",
  "ledger-config",
  "ledger-core",