pyvm: split op handlers into ops_core/ops_box/ops_ctrl; add ops_flow + intrinsic; delegate vm.py without behavior change

net-plugin: modularize constants (consts.rs) and sockets (sockets.rs); remove legacy commented socket code; fix unused imports
mir: move instruction unit tests to tests/mir_instruction_unit.rs (file lean-up); no semantic changes
runner/pyvm: ensure using pre-strip; misc docs updates

Build: cargo build ok; legacy cfg warnings remain as before

docs/guides/exception-handling.md

@@ -0,0 +1,88 @@
+Exception Handling — Postfix catch / cleanup (Stage‑3)
+
+Summary
+- Nyash adopts a flatter, postfix-first exception style:
+  - try is deprecated. Use postfix `catch` and `cleanup` instead.
+  - `catch` = handle exceptions from the immediately preceding expression/call.
+  - `cleanup` = always-run finalization (formerly finally), regardless of success or failure.
+- This matches the language’s scope unification and keeps blocks shallow and readable.
+
+Status
+- Phase 1: normalization sugar（既存）
+  - `NYASH_CATCH_NEW=1` でコア正規化パスが有効化。
+  - 後置フォームは内部 `TryCatch` AST に変換され、既存経路で降下。
+  - 実行時コストはゼロ（意味論不変）。
+- Phase 2（実装済み・Stage‑3ゲート）
+  - パーサが式レベルの後置 `catch/cleanup` を直接受理。
+  - ゲート: `NYASH_PARSER_STAGE3=1`
+  - 糖衣正規化はそのまま併存（関数糖衣専用）。キーワード直受理と二重適用はしない設計。
+
+Syntax (postfix)
+- Expression-level postfix handlers (Stage‑3):
+  - `expr catch(Type e) { /* handle */ }`
+  - `expr catch { /* handle (no variable) */ }`
+  - `expr cleanup { /* always-run */ }`
+  - Combine: `expr catch(Type e){...} cleanup{...}`
+- Method/function calls are just expressions, so postfix applies:
+  - `call(arg1, arg2) catch(Error e) { log(e) }`
+  - `obj.method(x) cleanup { obj.release() }`
+
+Precedence and chaining
+- Postfix `catch`/`cleanup` binds to the immediately preceding expression (call/chain result), not to the whole statement.
+- For long chains, we recommend parentheses to make intent explicit:
+  - `(obj.m1().m2()) catch { ... }`
+  - `f(a, b) catch { ... } cleanup { ... }`
+- Parser rule (Stage‑3): postfix attaches once at the end of a call/chain and stops further chaining on that expression.
+
+Diagram (conceptual)
+```
+// before (parse)
+obj.m1().m2() catch { H } cleanup { C }
+
+// precedence (binding)
+obj.m1().[ m2()  ↖ binds to this call ] catch { H } cleanup { C }
+
+// normalization (conceptual AST)
+TryCatch {
+  try:   [ obj.m1().m2() ],
+  catch: [ (type:Any, var:None) -> H ],
+  finally: [ C ]
+}
+```
+
+Normalization (Phase 1)
+- With `NYASH_CATCH_NEW=1`, postfix sugar is transformed into legacy `TryCatch` AST:
+  - `EXPR catch(T e){B}` → `TryCatch { try_body:[EXPR], catch:[(T,e,B)], finally:None }`
+  - `EXPR cleanup {B}` → `TryCatch { try_body:[EXPR], catch:[], finally:Some(B) }`
+  - Multiple `catch` are ordered top-to-bottom; first matching type handles the error.
+  - Combined `catch ... cleanup ...` expands to a single `TryCatch` with both blocks.
+- Lowering uses the existing builder (`cf_try_catch`) which already supports cleanup semantics.
+
+Semantics
+- catch handles exceptions from the immediately preceding expression only.
+- cleanup is always executed regardless of success/failure (formerly finally).
+- Multiple catch blocks match by type in order; the first match is taken.
+- In loops, `break/continue` cooperate with cleanup: cleanup is run before leaving the scope.
+
+Migration notes
+- try is deprecated: prefer postfix `catch/cleanup`.
+- Member-level handlers (computed/once/birth_once/method) keep allowing postfix `catch/cleanup` (Stage‑3), unchanged.
+- Parser acceptance of postfix at expression level will land in Phase 2; until then use the gate for normalization.
+
+Examples
+```
+// Postfix catch on a call
+do_work() catch(Error e) { env.console.log("error: " + e) }
+
+// Always-run cleanup
+open_file(path) cleanup { env.console.log("closed") }
+
+// Combined
+connect(url)
+  catch(NetworkError e) { env.console.warn(e) }
+  cleanup { env.console.log("done") }
+
+// Stage‑3 parser gate quick smoke (direct acceptance)
+//   NYASH_PARSER_STAGE3=1 ./target/release/nyash --backend vm \
+//     apps/tests/macro/exception/expr_postfix_direct.nyash
+```