docs: keep engine and transfer-core comments consumer-agnostic

[idas0]

What & why

Doc-comment and docs-only pass over the engine, the PyO3 bindings, and the transfer core: reword comments that described one deployment's architecture in terms of the generic mechanism (producer/caller rather than named pipeline roles, encode-time rather than transfer framing), and drop references to internal planning notes that don't belong in code comments. Trims one matching sentence in the v1.3.2 CHANGELOG entry. No code, API, or contract changes — every documented engine invariant stays documented.

How it was tested

• Comments/docs-only diff (44+/49− across 12 files); no runtime code touched.
• uv run pytest tests/test_segments.py -q — 9 passed (covers the touched Python files).
• uv run ruff check on the touched Python files — clean.

Ground rules (see CONTRIBUTING.md)

• No raw documents / queries / embeddings / credentials added to logs, telemetry, or the redacted artifacts — comments only
• Runtime deps unchanged; tests/test_import_boundary.py unaffected
• Licensing preserved (LOCAL_PATCHES.md wording only; the vendored MIT notice is untouched)
• No behavior change — existing suites cover every touched file