Add reduce-allocs skill and string builder infrastructure (#937)

Adds infrastructure for reducing short-lived heap allocations, to be applied across the codebase in follow-up PRs.

- **`reduce-allocs` Claude Code skill** — reviews code for unnecessary heap allocations and suggests fixes using stack-friendly patterns (`ExtendableStringBuilder`, `eastl::fixed_vector`, `TRefCounted`, etc.)
- **`TransparentStringHash`** (`zencore/hashutils.h`) — enables `std::string_view` lookups on `std::string`-keyed `unordered_map` without allocating a temporary string (C++20 heterogeneous lookup via `is_transparent`)
- **`AppendPaddedInt()`** and **`AppendFill()`** on `StringBuilderBase` (`zencore/string.h`) — zero-padded integer formatting and repeated-character fills without going through `fmt::format`
- **`StringBuilderAppender`** output iterator adapter — allows `fmt::format_to` to write directly into a `StringBuilderBase`

1 files changed, 34 insertions, 0 deletions

diff --git a/src/zencore/string.cpp b/src/zencore/string.cpp
index df7d71250..44f78aa75 100644
--- a/src/zencore/string.cpp
+++ b/src/zencore/string.cpp
@@ -543,6 +543,40 @@ template class StringBuilderImpl<wchar_t>;
 //////////////////////////////////////////////////////////////////////////
 
 void
+StringBuilderBase::AppendPaddedInt(int64_t Value, int MinWidth)
+{
+	char	 Buf[24];
+	char*	 End	  = Buf + sizeof(Buf);
+	char*	 Ptr	  = End;
+	bool	 Negative = Value < 0;
+	uint64_t Abs	  = Negative ? uint64_t(-Value) : uint64_t(Value);
+	do
+	{
+		*--Ptr = '0' + char(Abs % 10);
+		Abs /= 10;
+	} while (Abs > 0);
+	while ((End - Ptr) < MinWidth)
+	{
+		*--Ptr = '0';
+	}
+	if (Negative)
+	{
+		*--Ptr = '-';
+	}
+	AppendRange(Ptr, End);
+}
+
+void
+StringBuilderBase::AppendFill(char C, size_t Count)
+{
+	EnsureCapacity(Count);
+	std::memset(m_CurPos, C, Count);
+	m_CurPos += Count;
+}
+
+//////////////////////////////////////////////////////////////////////////
+
+void
 UrlDecode(std::string_view InUrl, StringBuilderBase& OutUrl)
 {
 	std::string_view::size_type i = 0;