fix(dx): Quick Win 1-3 for better error messages and API simplification

Quick Win 1: Show available boxes on "Unknown Box type" error
- vm.rs, vm_fallback.rs: Display sorted list of available user-defined boxes
- Before: "Unknown Box type: Foo"
- After:  "Unknown Box type: Foo. Available: Bar, Baz, Main"

Quick Win 2: Show stderr on child process timeout
- child.rs, selfhost_exe.rs: Capture and display stderr (up to 500 chars)
- Helps diagnose what went wrong in selfhost compiler child process

Quick Win 3: Simplify Stage-B compiler API (SSOT)
- compiler_stageb.hako: Add StageBDriverBox.compile() as single entry point
- compiler_stageb.hako: Remove StageBMain compatibility wrapper
- compiler.hako: Change from `using ... as StageBMain` to direct import
- compiler.hako: Call StageBDriverBox.compile() directly

Also includes child_env.rs NYASH_MODULES env var for module mapping.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

lang/src/compiler/entry/compiler.hako

@@ -2,7 +2,9 @@
 // - When invoked with --min-json, emit minimal Program JSON v0 to stdout
 // - Otherwise, act as a silent placeholder (return 0)
 
-using lang.compiler.entry.compiler_stageb as StageBMain
+// Phase 28.2 Quick Win 3: Direct import without alias
+// StageBDriverBox.compile() is the SSOT entry point for Stage-B compilation
+using lang.compiler.entry.compiler_stageb
 
 static box Main {
   _parse_signed_int(raw) {
@@ -478,7 +480,8 @@ static box Main {
   main(args) {
     local flags = me._collect_flags(args)
     if flags.stage_b == 1 {
-      local json = StageBMain._do_compile_stage_b(flags.source, flags.prefer_cfg, flags.stage3, flags.v1_compat)
+      // Phase 28.2 Quick Win 3: Direct call to SSOT
+      local json = StageBDriverBox.compile(flags.source, flags.prefer_cfg, flags.stage3, flags.v1_compat)
       print(json)
       return 0
     }

lang/src/compiler/entry/compiler_stageb.hako

@@ -1139,6 +1139,89 @@ static box StageBHelperBox {
 
 // Phase 25.1c: Main driver logic
 static box StageBDriverBox {
+  // Phase 28.2 Quick Win 3: SSOT compile API for compiler.hako direct call
+  // Returns Program(JSON v0) instead of printing it.
+  compile(source, prefer_cfg, stage3, v1_compat) {
+    // 1) Set env vars for internal functions
+    if stage3 != null && stage3 != 0 {
+      env.set("NYASH_PARSER_STAGE3", "1")
+      env.set("HAKO_PARSER_STAGE3", "1")
+    }
+
+    // 2) Create parser
+    local p = new ParserBox()
+    if stage3 != null && stage3 != 0 {
+      p.stage3_enable(1)
+    }
+
+    // 3) Extract body and parse
+    local body_src = StageBBodyExtractorBox.build_body_src(source, null)
+
+    // 4) Parse to JSON
+    local ast_json = "{\"version\":0,\"kind\":\"Program\",\"body\":[]}"
+    if body_src != null {
+      local block_src = "{" + body_src + "}"
+      local block_res = p.parse_block2(block_src, 0)
+      local at = block_res.lastIndexOf("@")
+      if at >= 0 {
+        local body_json = block_res.substring(0, at)
+        if body_json != null && body_json != "" {
+          ast_json = "{\"version\":0,\"kind\":\"Program\",\"body\":" + body_json + "}"
+        }
+      }
+    }
+
+    // 5) Apply SSA transformations
+    ast_json = CompilerBuilder.apply_all(ast_json)
+
+    // 6) Scan for function definitions
+    local defs_json = ""
+    local methods = StageBFuncScannerBox.scan_all_boxes(source)
+    if methods != null && methods.length() > 0 {
+      defs_json = ",\"defs\":["
+      local mi = 0
+      local mn = methods.length()
+      loop(mi < mn) {
+        local def = methods.get(mi)
+        local mname = "" + def.get("name")
+        local mparams = def.get("params")
+        local mbody = "" + def.get("body_json")
+        local mbox = "" + def.get("box")
+        local wrapped_body = "{\"type\":\"Block\",\"body\":" + mbody + "}"
+        local params_arr = "["
+        local pi = 0
+        local pn = mparams.length()
+        loop(pi < pn) {
+          if pi > 0 { params_arr = params_arr + "," }
+          params_arr = params_arr + "\"" + ("" + mparams.get(pi)) + "\""
+          pi = pi + 1
+        }
+        params_arr = params_arr + "]"
+        if mi > 0 { defs_json = defs_json + "," }
+        defs_json = defs_json + "{\"name\":\"" + mname + "\",\"params\":" + params_arr + ",\"body\":" + wrapped_body + ",\"box\":\"" + mbox + "\"}"
+        mi = mi + 1
+      }
+      defs_json = defs_json + "]"
+    }
+
+    // 7) Inject defs into Program JSON
+    if defs_json != "" && defs_json.length() > 0 {
+      local ajson = "" + ast_json
+      local close_pos = -1
+      local j = ajson.length() - 1
+      loop(j >= 0) {
+        if ajson.substring(j, j + 1) == "}" { close_pos = j  break }
+        j = j - 1
+      }
+      if close_pos >= 0 {
+        ast_json = ajson.substring(0, close_pos) + defs_json + ajson.substring(close_pos, ajson.length())
+      }
+    }
+
+    // 8) Return JSON (SSOT - single return point)
+    return ast_json
+  }
+
   main(args) {
     // ============================================================================
     // Phase 25.1c: Guaranteed marker for entry point confirmation (dev-only)