OmniNet 0.7.3: Documentation updates for v0.7.1 + v0.7.2

No code changes. Docs-only release covering the 11 bug fixes
shipped in 0.7.1 and 0.7.2.

  - docs/migration/v0.7.0-to-v0.7.2.md (NEW)
    Full migration guide covering both intermediate releases.
    Compatibility matrix, recommended actions with before/after
    code, security defaults, performance notes.

  - docs/api/http-server.md
    Updated CORS Setup with the new allowlist behavior table.
    Updated CORS Headers with per-request Origin logic.
    Added new TLS Client Connections section covering
    TLSContext::createClient/createServer/createSocket,
    security defaults, sharing contexts, ALPN usage.

  - docs/api/index.md
    Added What's New section linking to migration guide.
    Summarizes v0.7.2 TLS fixes, v0.7.1 bug fixes,
    v0.7.0 WebSocket fix.

  - docs/api/websocket.md
    Added note about registerProtocolSwitch() atomic
    registration (the v0.7.0 fix for TOCTOU race and
    single-shot erase).

README.md untouched — it has no specific class/version
references, and detail belongs in the API docs.

Build verified clean (no code changes anyway).