Add a ResponseHandler<T> seam with lazy parse-once ParsedResponse<T>

[OmarAlJarrah]

Problem

There's no clean raw-vs-parsed response seam; header/status access always sits in front of (or forces) deserialization.

Proposed change

Add a fun interface ResponseHandler<T> plus ParsedResponse<T> (or Response.parseable { }) with by lazy parse-once memoization. Ship dep-free String/Empty handlers in core; the JSON handler (jsonHandler(serde, Class<T>)) lives in the adapter. Document that parse() consumes/closes the body, so raw headers must be read first. This is the seam generated services dispatch against.

Acceptance

• Raw header/status access without parsing
• Parse-once memoized
• Tests + apiDump

Dependencies

• Blocks Generate two-tier raw/cooked service methods #55 — two-tier raw/cooked services
• Blocks Generate a per-endpoint SSE adapter mapping events to typed models #62 — per-endpoint SSE adapter

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Priority: medium · Effort: small