feat(decompiler): Rec 31 #31-3 RAII Stage 2B — XmlScan::lvalue unique_ptr migration

[CryptoJones]

Converts XmlScan's per-token string buffer from a raw owning pointer to unique_ptr<string>. This eliminates 7 raw new string() allocation sites (one per scan* mode helper) and the manual delete in ~XmlScan / clearlvalue, replacing them with make_unique + the unique_ptr destructor's automatic cleanup. The single ownership-transfer point (lval(), called by bison's yylex to hand the string up to the parser value stack) uses unique_ptr::release() so the bison side's existing raw-pointer ownership convention is unchanged.

Before:

class XmlScan {
  string *lvalue;                // raw owning pointer
  ...
  string *lval(void) {
    string *ret = lvalue;
    lvalue = (string *)0;        // manual null-out
    return ret;
  }
  void clearlvalue(void) {
    if (lvalue != (string *)0)   // manual null-check
      delete lvalue;             // manual delete
  }
};
lvalue = new string();           // ×7 across scan* helpers

After:

class XmlScan {
  unique_ptr<string> lvalue;     // owning RAII handle
  ...
  string *lval(void) {
    return lvalue.release();     // ownership transfer; lvalue → nullptr
  }
  void clearlvalue(void) {
    lvalue.reset();              // null-safe, automatic delete
  }
};
lvalue = make_unique<string>();  // ×7

Bison sync: xml.y is the grammar source; xml.cc is the generated parser. The edits are entirely in the epilogue / prologue %{...%} regions that bison copies verbatim, so both files get the identical hand-edit and stay in lockstep without invoking bison. The repo's xml.cc was generated by bison 3.0.4; the local box has 3.8.2 and a clean regeneration would produce huge unrelated diff — keeping the parallel hand-edit avoids that. xml.hh picks up <memory> and the make_unique / move / unique_ptr usings alongside the existing std:: aliases.

Scope intentionally limited:

• Semantic-action raw news (xml.y:150, 153, 198, 200, 208 and the parallel sites in xml.cc's yyparse) are NOT touched in this PR. They live inside bison's table-driven yyparse and a clean migration requires actual bison-3.0.4 regeneration; that's a separate follow-up PR (Stage 2C).
• The big-object epilogue raw news (line 525 new XmlScan, line 538 new Element, line 624 new Document) require API redesign across xml.hh + callers and are separate PRs of their own.
• cppRaiiAudit's PROTECTED_FILES does NOT yet include xml.cc / xml.hh / xml.y — adding them now would fail the gate on the still-present semantic-action raw news. Add after Stage 2C lands.

Tests: this is functionally a no-op refactor. The behavior change is "manual delete replaced by unique_ptr destructor"; both forms free the string on exactly the same code paths. Marshal RAII Stage 2A (PR #46) had the same shape and passed C++ unit tests + ASan + UBSan.

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