feat(testing): line coverage via --coverage + bin/x test --coverage (#45)

Instrument each emitted statement with a per-source-line hit counter, gated
behind a new --coverage flag (default off) so ordinary builds — and the
compiler's own self-compile — stay byte-identical and the C-free bootstrap
fixpoint is untouched. A static line table plus a parallel hit-counter array
are dumped at exit through an atexit hook to $LUDIC_COVERAGE (default
ludic.cov) as a `FILE <name>` header and `<line> <hits>` rows.

bin/x test --coverage compiles the test specs with instrumentation, runs them
into per-file dumps, and aggregates a clean per-file line-coverage report that
names the unreached lines. Adds examples/library/coverage.ludic (a spec whose
tests deliberately miss one branch) and docs. Closes the last open acceptance
item of the testing framework (#12).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/backend/emit_stmt.ludic

@@ -222,6 +222,7 @@ function emit_emit(st: Node) -> Val {
 }
 
 function emit_stmt(st: Node) -> void {
+  emit_cov_hit(st.line)                            # --coverage: bump this line's hit counter (no-op otherwise)
   if st.kind == S_LET {
     var ty = st.ty
     if (ty == null) { let v0 = emit_expr(st.a);  ty = v0.ty