Document static binary architecture decision

ober

0f69e08bd17a6c404423842793d24c1a326e8c42

diff --git a/GAPS.md b/GAPS.md
index 8f5d28d..047f7f0 100644
--- a/GAPS.md
+++ b/GAPS.md
@@ -479,6 +479,13 @@ Acceptance criteria:
 - Keep module boundaries aligned to Git, signals, aggregation, reports, config,
   and providers.
 
+Status: explicitly rejected for this parity milestone; see
+`docs/architecture.md`. The static binary pipeline is verified against
+`main-binary.ss`, and a source split would be a build-system refactor rather
+than detector/reference parity behavior. The intended module boundaries are
+documented for a future refactor that must first prove `make verify` and
+`make binary` do not regress.
+
 ### G-051: No golden JSON snapshots
 
 Current shell tests grep output. They do not preserve full stable report
diff --git a/docs/architecture.md b/docs/architecture.md
new file mode 100644
index 0000000..7dc7a73
--- /dev/null
+++ b/docs/architecture.md
@@ -0,0 +1,39 @@
+# Architecture decision: static-binary-first layout
+
+`jerboa-aigit` currently keeps the scanner implementation in `main-binary.ss`.
+This is intentional for the current parity milestone.
+
+The original plan proposed module boundaries for Git access, signals,
+aggregation, reports, config, and providers. Those boundaries are visible in
+the code organization and JSON/report contracts, but they are not split into
+separate Jerboa source modules yet.
+
+Reason:
+
+- The parity work now depends on a verified static binary pipeline.
+- `make binary` builds `main-binary.ss` directly with `jerbuild binary`.
+- Splitting Jerboa source files would be a packaging/build-system refactor, not
+  a detector parity feature.
+- The repository rule for G-050 says to split modules only when doing so does
+  not regress static binary builds. That condition has not been proven for this
+  project.
+
+Decision:
+
+Keep the single Jerboa entry file for this parity milestone. Treat a future
+module split as a separate build-system refactor that must preserve:
+
+- `bin/jerboa-aigit` CLI compatibility;
+- `make verify`;
+- `make binary`;
+- static binary behavior on macOS and the Linux release pipeline;
+- the current JSON output contract and golden snapshots.
+
+The intended future module boundaries remain:
+
+- Git/process/path handling;
+- note/provenance parsing;
+- signals and strategy catalog;
+- scoring/aggregation;
+- reports/JSON/markdown/stats;
+- config/provider adapters.