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/update.el

@@ -6,6 +6,15 @@
 // Depends on: registry.el (registry_latest_version, registry_find),
 //             install.el  (read_installed, install_vessel, installed_version)
 
+// ── Cross-module forward declarations ─────────────────────────────────────────
+// Defined in sibling epm modules; resolved at link time. The `extern fn` decls
+// give elc the C prototypes so generated update.c compiles cleanly under strict
+// compilers (gcc>=14 / clang) that reject implicit function declarations.
+extern fn read_installed() -> String                                  // install.el
+extern fn installed_version(name: String) -> String                   // install.el
+extern fn install_vessel(name: String, version: String) -> Bool      // install.el
+extern fn registry_latest_version(name: String) -> String            // registry.el
+
 // ── Semver helpers ────────────────────────────────────────────────────────────
 
 // semver_part extracts the Nth dot-separated component from a semver string.