feat(story-1.4): transport protocol and Httpx2Transport adapter

[lesnik512]

Adds the Transport protocol (Seam 1, middleware ↔ transport) and the default Httpx2Transport implementation — the single seam where httpx2 is imported. Adds a minimal StreamResponse placeholder so the protocol's stream() signature can be typed today and fleshed out in Story 4.1.

• src/httpware/transports/init.py: @runtime_checkable Transport Protocol with call / stream / aclose
• src/httpware/transports/httpx2.py: Httpx2Transport adapter; lazy AsyncClient init guarded by asyncio.Lock; httpx2.{TimeoutException, HTTPError, InvalidURL, CookieConflict} and the closed-client RuntimeError mapped to httpware.{TimeoutError, TransportError} with the original message preserved on the mapped instance
• src/httpware/response.py: StreamResponse placeholder (status, headers, url) re-exported from httpware/init.py
• tests/test_no_httpx2_leakage.py: CI invariant — only transports/httpx2.py may import httpx2; cwd-robust pathing
• tests/test_transports_httpx2.py: 63 unit tests covering protocol membership, success / 4xx / 5xx / exception-mapping / lazy-init / concurrent-init / post-close paths
• docs/stories/1-4-...: story spec + Review Findings with 3-layer adversarial review outcome
• docs/deferred-work.md: 5 review-time deferrals + extended case-insensitive-headers entry to cover multi-value collapse
• CHANGELOG.md: Unreleased / Added entry

Full suite 140 passing, 100% coverage on every source module, lint green (ruff + ty), httpx2 leakage invariant holds.