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/blueprints/strings-utf8-byte.md

@@ -0,0 +1,61 @@
+# Strings Blueprint — UTF‑8 First, Bytes Separate
+
+Status: active (Phase Freeze compatible)
+Updated: 2025-09-21
+
+Purpose
+- Unify string semantics by delegating StringBox public APIs to dedicated cursor boxes.
+- Keep behavior stable while making codepoint vs byte decisions explicit and testable.
+
+Pillars
+- Utf8CursorBox (codepoint-oriented)
+  - length/indexOf/substring operate on UTF‑8 codepoints.
+  - Intended as the default delegate for StringBox public APIs.
+- ByteCursorBox (byte-oriented)
+  - length/indexOf/substring operate on raw bytes.
+  - Use explicitly for byte-level parsing or binary protocols.
+
+Delegation Strategy
+- StringBox delegates to Utf8CursorBox for core methods: length/indexOf/substring.
+- Provide conversion helpers: toUtf8Cursor(), toByteCursor() (thin wrappers).
+- Prefer delegation over inheritance; keep “from” minimal to avoid API ambiguity.
+
+API Semantics
+- indexOf: define two flavors via the box boundary.
+  - StringBox.indexOf → Utf8CursorBox.indexOf (CP-based; canonical)
+  - ByteCursorBox.indexOf → byte-based; opt‑in only
+- substring: follow the same split (CP vs Byte). Do not mix semantics.
+  - Document preconditions for indices (out‑of‑range clamped/errored per guide).
+
+Implementation Plan (staged, non‑breaking)
+1) Provide MVP cursor boxes (done)
+   - apps/libs/utf8_cursor.nyash
+   - apps/libs/byte_cursor.nyash
+2) Delegate StringBox public methods to Utf8CursorBox (internal only; behavior unchanged)
+   - Start with length → indexOf → substring
+   - Add targeted smokes for edge cases (multi‑byte CP, boundaries)
+3) Replace ad‑hoc scans in Nyash scripts with cursor usage (Mini‑VM/macros)
+   - Migrate internal scanners (no external behavior change)
+4) Introduce ByteCursorBox only where byte‑level semantics are required
+   - Keep call sites explicit to avoid ambiguity
+
+Transition Gate (Rust dev only)
+- Env `NYASH_STR_CP=1` enables CP semantics for legacy byte-based paths in Rust runtime (e.g., StringBox.length/indexOf/lastIndexOf).
+- Default remains byte in Rust during freeze; PyVM follows CP semantics. CI smokes validate CP behavior via PyVM.
+
+Related Docs
+- reference/language/strings.md — policy & scope
+- guides/language-core-and-sugar.md — core minimal + sugar
+- reference/language/EBNF.md — operators (! adopted; do‑while not adopted)
+- guides/loopform.md — loop normalization policy
+
+Box Foundations (string-related)
+- Utf8CursorBox, ByteCursorBox
+- StringExtBox (trim/startsWith/endsWith/replace/split)
+- StringBuilderBox (append/toString)
+- JsonCursorBox (lightweight JSON scanning helpers)
+
+Testing Notes
+- Keep PyVM as the reference execution path.
+- Add smokes: CP boundaries, mixed ASCII/non‑ASCII, indexOf not found, substring slices.
+- Avoid perf work; focus on semantics + observability.