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>

docs/language/testing/kw-test.md

@@ -19,6 +19,16 @@ Assertions (each records a failure and prints <code>file:line: … failed</code>
 
 Run a spec file directly with the compiler-runner — `ludic mymath_test.ludic` compiles it to a native binary, runs it, and forwards the pass/fail exit code — so it drops straight into `bin/x` and CI.
 
+<strong>Line coverage.</strong> Compile with the <code>--coverage</code> flag and the compiler instruments every statement with a per-source-line hit counter; at exit the counts are written to the file named by <code>$LUDIC_COVERAGE</code> (default <code>ludic.cov</code>) as a <code>FILE &lt;name&gt;</code> header followed by one <code>&lt;line&gt; &lt;hits&gt;</code> row per instrumented line. The instrumentation is flag-gated and additive, so an ordinary build — and the compiler's own self-compile — stays byte-identical. <code>bin/x test --coverage</code> compiles the test specs this way, runs them, and aggregates the dumps into a per-file report that names the lines your tests never reached:
+
+```
+== line coverage (bin/x test --coverage) ==
+  examples/library/coverage.ludic   16/17 lines  94%   uncovered: 33
+  examples/library/testing.ludic    17/17 lines  100%
+  ----
+  total: 33/34 lines  97%
+```
+
 ```ludic
 program MathSpec {
   function add(a: int, b: int) -> int { return a + b }