Introduce a dedicated device_attributes.zig module that consolidates
all device attribute types and encoding logic. This moves
DeviceAttributeReq out of ansi.zig and adds structured response
types for DA1 (primary), DA2 (secondary), and DA3 (tertiary) with
self-encoding methods.
Primary DA uses a ConformanceLevel enum covering VT100-series
per-model values and VT200+ conformance levels, plus a Feature
enum with all known xterm DA1 attribute codes (132-col, printer,
sixel, color, clipboard, etc.) as a simple slice. Secondary DA
uses a DeviceType enum matching the xterm decTerminalID values.
Tertiary DA encodes the DECRPTUI unit ID as a u32 formatted to
8 hex digits.
This is preparatory work for exposing device attributes through
the stream_terminal Effects callback system.
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