refactor: extract socket loop into its own module, move e2e tests to tests/

[zheylmun]

Split lib.rs along its two natural seams. lib.rs now holds only the public
Socketeer handle (connect/send/receive/reconnect/close); the background task
that owns the split WebSocket stream moves to src/socket_loop.rs. The handle's
entire contract with the loop is now expressed through three pub(crate) items
(TxChannelPayload, send_close, socket_loop_split) plus the shared
WebSocketStreamType alias.

The ~580 lines of end-to-end tests, which only ever touched the public API and
the mocking exports, move to tests/integration.rs as integration tests. The
genuine unit tests in codec.rs stay inline.

Also folds in the trace-level normalization discussed alongside this work,
since those lines were relocated by the extraction: routine keepalive sends
info -> trace, per-message send debug -> trace (matching the receive side), and
the expected reconnect close-failure error -> debug.

No behavior change. All tests pass (15 unit, 25 integration, 5 doc); clippy
pedantic and rustfmt clean.

[codecov]

Codecov Report

❌ Patch coverage is 97.61905% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.05%. Comparing base (ad2be98) to head (b7ec286).

Files with missing lines	Patch %	Lines
src/socket_loop.rs	97.61%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
- Coverage   98.52%   97.05%   -1.47%     
==========================================
  Files           5        6       +1     
  Lines         811      408     -403     
==========================================
- Hits          799      396     -403     
  Misses         12       12              

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