compiler: a missing import is an error, not an empty string

import "../../NOPE/does_not_exist.el"

compiled CLEANLY — exit 0, empty stderr, and a program silently missing
everything it imported.

resolve_imports did `fs_read(src_path)` and used the result without checking.
fs_read returns "" both for "file is empty" and "file does not exist", so a
typo, a moved file, or a relative path resolved from the wrong working
directory all produced a successful build of nothing.

It caused a real wrong conclusion during test-framework work: a bisection run
from a subdirectory where ../../runtime/ did not resolve produced ELEVEN
consecutive "successful" compiles that had included no runtime at all, and the
results were believed before anyone noticed.

Missing dependency, confident success — the same shape as a test suite
reporting pass for tests that never ran, and as a benchmark reporting 0us
because the optimiser deleted the loop.

fs_exists separates the two cases, so a legitimately empty file still resolves
to "" and is fine. A path that does not exist now prints the resolved path and
exits 1, which is what build scripts check.

Verified:
  - bad import: exit 1 (was 0), message names the resolved path
  - elc-cli.el still compiles, self-hosting fixpoint byte-identical
  - neuron's full soul amalgam regeneration: exit 0, 405ms, output
    byte-identical at 1,270,212 bytes

lang/el-compiler/src/compiler.el

@@ -419,6 +419,22 @@ fn resolve_imports(src_path: String) -> String {
     if !str_eq(already, "") { return "" }
     state_set(seen_key, "1")
 
+    // A missing file must be a hard error, never an empty string.
+    //
+    // fs_read returns "" both for "file is empty" and "file does not exist", and
+    // this function used the value without distinguishing them. So a broken
+    // import path — a typo, a moved file, a relative path resolved from the
+    // wrong working directory — compiled CLEANLY: exit 0, empty stderr, and a
+    // program silently missing everything it imported. Observed 2026-08-15:
+    // eleven consecutive "successful" compiles that had included no runtime at
+    // all, and a wrong conclusion drawn from them before anyone noticed.
+    //
+    // Missing dependency, confident success. fs_exists separates the two cases,
+    // so a genuinely empty file still resolves to "" and is fine.
+    if !fs_exists(src_path) {
+        println("elc: cannot resolve import: " + src_path)
+        exit_program(1)
+    }
     let source: String = fs_read(src_path)
     let dir: String = dirname_of(src_path)
     let lines: [String] = str_split(source, "\n")