feat(decompiler): Rec 31 #31-3 Stage 2C-min — xml.y content-Reference tmp to stack-local

[CryptoJones]

Stage 2C-min step 1 from docs/decompiler/RAII_STAGE_2C_XML.md.

The semantic action for the content Reference rule allocated a temporary string on the heap, appended a single int4 to it, passed it by reference to print_content, then immediately deleted it:

| content Reference {
    string *tmp = new string();
    *tmp += $2;
    print_content(*tmp);
    delete tmp;
    global_scan->setmode(XmlScan::CharDataMode);
  }

The temporary never escapes its enclosing block — pure code-smell. Replacement is a stack-local:

| content Reference {
    string tmp;
    tmp += $2;
    print_content(tmp);
    global_scan->setmode(XmlScan::CharDataMode);
  }

print_content takes const string &, so passing the stack-local by reference is identical to dereferencing the previous raw owner. No bison regeneration needed: the change is inside a semantic action body (which bison copies verbatim into yyparse's switch); xml.y and xml.cc:1790 get the same hand-edit and stay in sync.

Does NOT touch the five other bison-mediated raw-new sites (xml.y:150, 153, 198, 200 and the parallel sites in xml.cc) — those need the bison %union → %define api.value.type variant migration scoped in the design doc, which is a strategic sprint of its own.

Does NOT add xml.y / xml.cc to cppRaiiAudit's PROTECTED_FILES yet — would require enhancing the audit task to accept per-file line-range exclusions for the five bison-mediated sites. Tracked in the design doc as deferred.

CHANGELOG.md [Unreleased] section updated to record Stage 2C-min shipped, per the per-PR changelog rule.

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