Point the build docs at the glibc check script

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013kj58Jugkic783b8ZrtTVZ

docs/REPOS.md

@@ -161,14 +161,22 @@ on a newer userland still fails to load on an older one. A JetPack or Ubuntu
 upgrade applied to one unit and not the other reintroduces the problem, silently
 and only at runtime. **Upgrade both, or neither.**
 
-The check that catches drift between them:
+The check that catches drift between them is `scripts/check-glibc.sh` in each
+service repo:
 
 ```bash
-objdump -T <binary> | grep -o 'GLIBC_[0-9.]*' | sort -Vu | tail -1
+scripts/check-glibc.sh target/release/svc-nav        # ceiling defaults to 2.35
 ```
 
-It detects, it does not fix — its job is to fail the build rather than let a bad
-binary reach the car, and to name the symbol that caused it.
+| Exit | Meaning |
+|---|---|
+| 0 | Highest referenced version is at or below the ceiling |
+| 1 | Ceiling exceeded — the responsible symbols are printed |
+| 2 | **Could not check** — no `objdump`, missing binary, no versions found |
+
+It detects, it does not fix; the fix is building on a unit whose release matches
+the car's. Exit 2 is distinct on purpose: a check that cannot check must not
+report success.
 
 ##### Cross-compiling from the workstation