add .ssp regression test script and reference files

test/test-abstraction-ssp.sh: shell script that generates
.ssp files for all sample documents and diffs against
committed reference files in test/reference/abstraction/.

Usage:
  ./test/test-abstraction-ssp.sh --generate ./bin/spine-ldc
  ./test/test-abstraction-ssp.sh ./bin/spine-ldc

Exits 0 if all .ssp files match, 1 if differences found.
Reports changed, missing, and new files.

test/reference/abstraction/: 35 reference .ssp files
generated from the current abstraction, covering all
sample documents including 9-language live-manual.

Co-Authored-By: Anthropic Claude Opus 4.6 (1M context)

1 files changed, 3 insertions, 3 deletions

diff --git a/flake.lock b/flake.lock
index aeaac22..4b8e88e 100644
--- a/flake.lock
+++ b/flake.lock
@@ -2,11 +2,11 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1775793324,
-        "narHash": "sha256-omax7atcZbol+6HJ2RLpP+ZCFcPa5bZ65Hn71RufeWQ=",
+        "lastModified": 1776329215,
+        "narHash": "sha256-a8BYi3mzoJ/AcJP8UldOx8emoPRLeWqALZWu4ZvjPXw=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "9d29d5f667d7467f98efc31881e824fa586c927e",
+        "rev": "b86751bc4085f48661017fa226dee99fab6c651b",
         "type": "github"
       },
       "original": {