wasm32 and wasm64: default library borrow facts, -mm=gc refused, a JS host and test runs

docs/how/wasm/test/.gitignore

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+out*/
+wasm/

docs/how/wasm/test/README.md

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+# Running tslang programs and tests as WebAssembly
+
+`wasm32-unknown-unknown` and `wasm64-unknown-unknown` have no C library. tslang links a wasm program
+with `--allow-undefined`, so whatever the C library and the default library's C++ wrappers provide on
+other targets (`malloc`, `snprintf`, `puts`, `regexp_*`, `stdio_write`, the time functions, ...) is an
+import from the `env` module. [host.mjs](host.mjs) implements them in JavaScript, for Node and for
+the browser.
+
+## The default library
+
+Built by the default library's `scripts\build_wasm.bat` (Windows) or `scripts/build_wasm.sh` (Linux)
+for `-mm=none`, `-mm=rc` and `-mm=own`, release and debug:
+
+    __build\defaultlib\lib\<wasm32|wasm64>\unknown\unknown\unknown\<release|debug>\<model>\libTypeScriptDefaultLib.a
+
+It is `lib.ts` plus the OS layer `lib.wasm.ts`, with no C++ wrappers. `-mm=gc` (tslang's default) is
+an error for a wasm triple, because the collector has no wasm build. Pass `-mm` explicitly.
+
+## One program
+
+    tslang --emit=exe -mm=rc --opt -mtriple=wasm32-unknown-unknown --default-lib-path=<...>\__build -o=hello.wasm hello.ts
+    node run.mjs hello.wasm
+
+The exit code is the program's own (`main`'s result, or `exit()`'s argument), or 1 for a trap or an
+uncaught exception.
+
+wasm64 needs Node 24 or later. LLVM gives a wasm64 module 64-bit tables, which older V8 cannot
+instantiate.
+
+## The tests
+
+    node run_tests.mjs [--suite=tester,defaultlib] [--triple=wasm32,wasm64] [--mm=none,rc,own] [--debug] [-j<n>] [filter]
+
+For each triple and model, this compiles the tests and runs each program under Node. The summary
+and every failure are written to `out/results.txt`.
+
+- `tester`: every single-file test the tester's CMakeLists.txt runs, built as test-runner builds them
+  (`--no-default-lib`). A test passes when it exits with 0 and prints `done.`.
+- `defaultlib`: the default library's own tests (`TypeScriptCompilerDefaultLib/tests`), linked with
+  the wasm default library and run in that repository, with `tests/<name>.stdin` as standard input.
+  A test passes when it prints `ALL DONE`, writes nothing to stderr and exits with 0, as `tests.ps1`
+  checks.
+
+Some tests are skipped, each with its reason:
+
+- tests that catch an exception (`try`/`catch` does not catch on wasm yet; a throw ends the program)
+- async tests (the async runtime has no wasm build)
+- tests that use threads
+- default-library tests marked `// gc only`, `// linux only`, or (under own) `// not own`
+- the tests in [wasm_skips.txt](wasm_skips.txt)
+
+Under `-mm=own`, a test that doesn't compile for wasm is compiled again natively. If that fails too,
+the test is listed as NATIVE and not counted as a failure: it is something `-mm=own` cannot prove
+yet. Native ctest runs only a few tests under own, but the whole corpus under none and rc, so under
+those two models every wasm compile error counts as a failure.
+
+## In a browser
+
+Put the `.wasm` files in `wasm\`, serve this folder over HTTP (e.g. `npx http-server .`) and open
+[index.html](index.html). It runs each program with the same host.