epm: declare cross-module callees as extern fn so strict compilers accept generated C

epm's sibling modules (registry/install/update) call functions defined in other
modules and in the El runtime (config, read_installed, registry_find,
manifest_deps, manifest_name, registry_latest_version, registry_token,
install_vessel, installed_version) without importing them, so elc emits no C
prototype for those calls. gcc<=13 treated the resulting implicit declarations
as warnings; gcc>=14 and clang reject them as hard errors, which is why the
"Build epm" CI step fails and blocks the whole dev/stage pipeline.

Add `extern fn` forward declarations -- El's own separate-compilation mechanism
-- for each cross-module callee at the top of registry/install/update. This
gives elc the correct C prototype in every generated translation unit, so the
calls compile cleanly and still resolve at link time. Simply suppressing
-Wimplicit-function-declaration would be unsafe: an implicit int return
truncates the 64-bit pointer returns of config/registry_find into a latent
crash, so declaring the true signatures is the correct fix. Localized to epm;
touches neither elc nor the runtime.

epm/src/registry.el

@@ -14,6 +14,15 @@
 //   EPM_REGISTRY_ORG — org name that hosts vessel repos (default: neuron-technologies)
 //   EPM_TOKEN      — Gitea personal access token (required for publish)
 
+// ── Cross-module forward declarations ─────────────────────────────────────────
+// These symbols are defined in sibling epm modules or the El runtime and are
+// resolved at link time. The `extern fn` decls give elc the C prototype so the
+// generated registry.c compiles cleanly under strict compilers (gcc>=14 / clang)
+// that reject implicit function declarations. Signature arity must match the
+// definition; return/param types are informational (all lower to el_val_t).
+extern fn config(key: String) -> String          // El runtime builtin
+extern fn read_installed() -> String              // install.el
+
 // ── Config helpers ────────────────────────────────────────────────────────────
 
 // registry_api_url returns the Gitea API base URL with no trailing slash.