ci: add GitHub Actions workflow; runner: prefer PyVM in selfhost paths; stage3: add LLVM + bridge acceptance smokes; docs: update env flags

apps/selfhost-compiler/README.md

@@ -12,6 +12,7 @@ Run (behind flag)
 - `NYASH_USE_NY_COMPILER=1 target/release/nyash --backend vm <program.nyash>`
   - The runner writes the input to `tmp/ny_parser_input.ny` and invokes this program.
   - It captures a JSON v0 line from stdout and executes it via the JSON bridge.
+  - Stage‑3 syntax gate: set `NYASH_NY_COMPILER_STAGE3=1` to pass `--stage3` to the parser (accepts Break/Continue/Throw/Try in JSON v0).
 
 Notes
 - Early MVP emits a minimal JSON v0 (currently a placeholder: return 0). We will gradually wire lexer/parser/emitter.

apps/tests/stage3_throw_dead_branch.nyash

@@ -0,0 +1,5 @@
+if false {
+  throw 1
+}
+return 0
+

apps/tests/stage3_try_finally_basic.nyash

@@ -0,0 +1,9 @@
+try {
+  local x = 1
+} catch (Error e) {
+  local y = 2
+} finally {
+  local z = 3
+}
+return 0
+