public: publish selfhost snapshot to public repo (SSOT using + AST merge + JSON VM fixes)

- SSOT using profiles (aliases/packages via nyash.toml), AST prelude merge
- Parser/member guards; Builder pin/PHI and instance→function rewrite (dev on)
- VM refactors (handlers split) and JSON roundtrip/nested stabilization
- CURRENT_TASK.md updated with scope and acceptance criteria

Notes: dev-only guards remain togglable via env; no default behavior changes for prod.

src/backend/mir_interpreter/handlers/boxes_string.rs

@@ -0,0 +1,55 @@
+use super::*;
+use crate::box_trait::NyashBox;
+
+pub(super) fn try_handle_string_box(
+    this: &mut MirInterpreter,
+    dst: Option<ValueId>,
+    box_val: ValueId,
+    method: &str,
+    args: &[ValueId],
+) -> Result<bool, VMError> {
+        let recv = this.reg_load(box_val)?;
+        let recv_box_any: Box<dyn NyashBox> = match recv.clone() {
+            VMValue::BoxRef(b) => b.share_box(),
+            other => other.to_nyash_box(),
+        };
+        if let Some(sb) = recv_box_any
+            .as_any()
+            .downcast_ref::<crate::box_trait::StringBox>()
+        {
+            match method {
+                "length" => {
+                    let ret = sb.length();
+                    if let Some(d) = dst { this.regs.insert(d, VMValue::from_nyash_box(ret)); }
+                    return Ok(true);
+                }
+                "substring" => {
+                    if args.len() != 2 {
+                        return Err(VMError::InvalidInstruction(
+                            "substring expects 2 args (start, end)".into(),
+                        ));
+                    }
+                    let s_idx = this.reg_load(args[0])?.as_integer().unwrap_or(0);
+                    let e_idx = this.reg_load(args[1])?.as_integer().unwrap_or(0);
+                    let len = sb.value.chars().count() as i64;
+                    let start = s_idx.max(0).min(len) as usize;
+                    let end = e_idx.max(start as i64).min(len) as usize;
+                    let chars: Vec<char> = sb.value.chars().collect();
+                    let sub: String = chars[start..end].iter().collect();
+                    if let Some(d) = dst { this.regs.insert(d, VMValue::from_nyash_box(Box::new(crate::box_trait::StringBox::new(sub)))) ; }
+                    return Ok(true);
+                }
+                "concat" => {
+                    if args.len() != 1 {
+                        return Err(VMError::InvalidInstruction("concat expects 1 arg".into()));
+                    }
+                    let rhs = this.reg_load(args[0])?;
+                    let new_s = format!("{}{}", sb.value, rhs.to_string());
+                    if let Some(d) = dst { this.regs.insert(d, VMValue::from_nyash_box(Box::new(crate::box_trait::StringBox::new(new_s)))) ; }
+                    return Ok(true);
+                }
+                _ => {}
+            }
+        }
+        Ok(false)
+}