stb 头字段偏移常量表：消除 vm/compiler 两侧裸魔法数

- vm/Image.cpp：匿名空间 constexpr 偏移常量（kOffMagic..kOffModelId + 段内行偏移），
  from/model_id/model_matches/func_row/const_entry 全部替换
- compiler/Stb.h：同值 inline constexpr 表（Codegen 写侧 + Stb 读侧共用）
- compiler/Codegen.cpp assemble_image：put_* 头字段/行内偏移替换
- compiler/Stb.cpp StbView：读侧偏移替换
- 两侧 static_assert：头大小 = 72 + 型号标识、字段偏移单调、行内布局自洽

compiler/src/Stb.cpp

@@ -243,31 +243,31 @@ StbView StbView::from(const uint8_t* buf, size_t len) {
         v.err_ = "image too short";
         return v;
     }
-    if (get_le32(buf + 0) != kMagic) {
+    if (get_le32(buf + kOffMagic) != kMagic) {
         v.err_ = "bad magic";
         return v;
     }
-    if (get_le32(buf + 4) != kVersion) {
+    if (get_le32(buf + kOffVersion) != kVersion) {
         v.err_ = "bad version";
         return v;
     }
-    v.cycle_limit_ = get_le32(buf + 8);
-    v.dt_ms_ = get_le32(buf + 12);
-    v.project_hash_ = get_le64(buf + 16);
-    v.entry_fn_id_ = get_le32(buf + 24);
-    v.n_globals_ = get_le32(buf + 28);
-    v.n_consts_ = get_le32(buf + 44);
-    v.n_funcs_ = get_le32(buf + 48);
-    v.offset_code_ = get_le32(buf + 60);
-    v.offset_fb_ = get_le32(buf + 64);
-    v.offset_data_ = get_le32(buf + 68);
+    v.cycle_limit_ = get_le32(buf + kOffCycleLimit);
+    v.dt_ms_ = get_le32(buf + kOffDtMs);
+    v.project_hash_ = get_le64(buf + kOffProjectHash);
+    v.entry_fn_id_ = get_le32(buf + kOffEntryFnId);
+    v.n_globals_ = get_le32(buf + kOffNGlobals);
+    v.n_consts_ = get_le32(buf + kOffNConsts);
+    v.n_funcs_ = get_le32(buf + kOffNFuncs);
+    v.offset_code_ = get_le32(buf + kOffCode);
+    v.offset_fb_ = get_le32(buf + kOffFb);
+    v.offset_data_ = get_le32(buf + kOffData);
 
     // 段校验（12.13：文件尾 SHA-256[32] 在数据段之后）
     if (len < static_cast<size_t>(v.offset_data_) + kSha256Size) {
         v.err_ = "missing sha256 tail";
         return v;
     }
-    const uint64_t offs[5] = {get_le32(buf + 52), get_le32(buf + 56), v.offset_code_,
+    const uint64_t offs[5] = {get_le32(buf + kOffConst), get_le32(buf + kOffFuncs), v.offset_code_,
                               v.offset_fb_, v.offset_data_};
     for (int i = 0; i < 5; ++i) {
         if (offs[i] < kHeaderSize || offs[i] > len - kSha256Size) {
@@ -311,7 +311,7 @@ ConstEntry StbView::const_entry(size_t i) const {
     if (ok_ && i < n_consts_) {
         const uint8_t* p = buf_ + offs_of_const() + i * kConstEntrySize;
         e.tag = get_le32(p);
-        e.value = get_le64(p + 4);
+        e.value = get_le64(p + kConstValueOff);
     }
     return e;
 }
@@ -327,9 +327,9 @@ StbView::FuncRow StbView::func_row(size_t i) const {
     FuncRow r;
     if (ok_ && i < n_funcs_) {
         const uint8_t* p = buf_ + get_le32(buf_ + 56) + i * kFuncRowSize;
-        r.nregs = get_le32(p + 0);
-        r.code_offset = get_le32(p + 4);
-        r.code_len = get_le32(p + 8);
+        r.nregs = get_le32(p + kFuncNregsOff);
+        r.code_offset = get_le32(p + kFuncCodeOff);
+        r.code_len = get_le32(p + kFuncLenOff);
     }
     return r;
 }
@@ -359,7 +359,7 @@ std::string StbView::model_id() const {
     if (!ok_) {
         return "";
     }
-    std::string s(reinterpret_cast<const char*>(buf_ + 72), kModelIdSize);
+    std::string s(reinterpret_cast<const char*>(buf_ + kOffModelId), kModelIdSize);
     const size_t z = s.find('\0');
     if (z != std::string::npos) {
         s.resize(z);
@@ -371,7 +371,7 @@ std::string StbView::model_id() const {
 bool StbView::model_matches(const std::string& name, uint32_t version) const {
     char want[kModelIdSize];
     fill_model_id(name, version, want);
-    return std::memcmp(buf_ + 72, want, kModelIdSize) == 0;
+    return std::memcmp(buf_ + kOffModelId, want, kModelIdSize) == 0;
 }
 
 /// 文件尾 32 字节 SHA-256 校验（对文件尾之前全部内容重算；未 ok() 时返回 false）。