feat(testing): built-in test blocks + expect assertions, auto-run with pass/fail (#12)

A `test "name" { ... }` top-level block is discovered automatically and run by a
synthetic runner @main — no `entry` to write, nothing to register. Inside a test,
expect(cond) / expect_eq(a, b) / expect_near(a, b, tol) assert; on failure they
print `file:line: <what> failed (got G, want W)` and set a per-test fail flag but
keep going, so one run reports every failure. The runner prints `ok   - name` /
`FAIL - name` per test, a `== N passed, M failed ==` summary, and exits non-zero
if any test failed — so `ludic spec_test.ludic` drops straight into bin/x and CI.
expect_near carries the tolerance fixed-point / accumulated-integer game math need.

Frontend: new `test` keyword (parse_test -> N_TEST, collected in g_tests) and the
call node now carries its source line for the file:line messages. Backend:
emit_test_runner synthesises @fn__test_i bodies + the runner @main; the expect*
builtins lower to a branch-print-flag tail (emit_expect_fail). g_src_name (set in
main from the input path) supplies the filename. The compiler's own source has no
`test` blocks, so its self-compiled IR is unchanged and the C-free fixpoint holds.

- `test` wired into the vocabulary (ludic_syntax.h, JetBrains lexer, TextMate
  grammar) and documented (docs/language/testing/)
- examples/library/testing.ludic: a passing spec, guarded by a new spec_case in
  the regression suite (build, run, require exit 0 + the expected summary)

Coverage instrumentation (the biggest lift in #12) is left as a follow-up.

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

docs/language/testing/_section.md

@@ -0,0 +1,7 @@
+---
+id: testing
+title: Testing
+order: 34
+---
+
+A built-in testing framework in the spirit of Go's <code>go test</code>: tests live next to the code, run with one command, and report pass/fail — no harness to wire up. A <a href="kw-test"><code>test "name" { … }</code></a> block is discovered automatically and run by a synthetic entry point that prints <code>ok   - name</code> or <code>FAIL - name</code> for each, a <code>== N passed, M failed ==</code> summary, and exits non-zero if anything failed (so CI and the <code>bin/x</code> runner catch it). Inside a test, the <code>expect</code>, <code>expect_eq</code> and <code>expect_near</code> assertions check a condition and, on failure, print <code>file:line: … failed (got …, want …)</code> and mark the test failed — without aborting, so one run reports every failure. <code>expect_near</code> takes a tolerance, which is what fixed-point and accumulated-integer game math need. Everything is deterministic and compiles to a native binary, so a suite runs in the same C-free toolchain as the rest of Ludic. Coverage instrumentation is a planned follow-up. Related: <a href="kw-function"><code>function</code></a>, <a href="fn-print"><code>print</code></a>.