feat(test): ludic test <dir>, each test block in a process of its own

A directory stands for its *_test.ludic files and the files straight
inside any tests/ under it. The runner answers --list and runs one test
by name, and ludic test runs every block in a child process, so tests
no longer share globals. A failed expect names the file it is written
in (the path the compiler was given) and its line. Reseed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

LANGUAGE.md

@@ -1775,11 +1775,27 @@ ludic run                                # compile src/main.ludic and run it
 ludic build --headless                   # headless build (renders out.ppm; reads stdin)
 ludic test                               # compile and run the project's `test` blocks
 ludic test tests/math.ludic --test adds  # just the test named "adds" (-v: every result line)
+ludic test packages/ludic.base          # the test programs under a directory (a package's)
 
 ludicc app.ludic -o build/app            # the compiler directly: a native binary
 ludicc app.ludic --emit-llvm -o app.ll   # stop at LLVM IR
 ```
 
+A test program is a file of `test "name" { ... }` blocks with `expect(cond)`, `expect_eq(a, b)` and
+`expect_near(a, b, tol)` in them. `ludic test` finds `tests/*.ludic` and `src/**/*_test.ludic`;
+given a directory, it runs every `*_test.ludic` under it and every file straight inside a `tests/`
+directory under it. **Each test block runs in a process of its own**, so a global one test changes
+is back to its initial value in the next - no test depends on another having run, or not. A
+failed assertion names its file, as the compiler was given it, and its line:
+
+```
+pk/src/sums_test.ludic:6: expect_eq failed (got 4, want 5)
+FAIL - wrong
+```
+
+A test program's runner takes a test's name as its one argument, and `--list` to name them all -
+which is how `ludic test` runs them one at a time.
+
 `ludic` is the CLI (`ludic help`); `ludicc` is the compiler it drives, built from
 the IR seed by `bin/ludic-dev build-cli`. **[COMPILING.md](COMPILING.md) is the
 authoritative CLI reference** — the full flag set (`-o`, `--windowed`,