refactor(core): collapse ConversationState into typed Continuation

[seanbrar]

Summary

Continuation state previously round-tripped through the legacy _conversation_state dict on every turn (build_conversation_state → ConversationState → Continuation, and back via an adapters bridge), carrying a second, parallel versioning scheme. That is the "compatibility drag inside the core" the v2 interaction sketch forbids, and it sat on the agent loop's hot path. This collapses it into a single typed builder, and folds in two small, related fixes.

• Replace the dict shape + adapters bridge with one typed build_continuation() that assembles the next Continuation from typed Message/Continuation objects directly.
• Delete src/pollux/continuation.py and src/pollux/interaction/adapters.py (~260 lines); move history_text_from_parts into parts.py. Net −164 lines, one conceptual layer removed. The provider boundary already consumed typed primitives, so provider replay is untouched.
• completion_status: unrecognized finish reasons now read "clean" instead of "cutoff" (removes the dead _CUTOFF_FINISH branch and unused _CLEAN_FINISH set), so a harness won't compact/retry on an unknown-but-successful stop.
• Remove stale Slice N / v1 build-process references from docstrings (no longer accurate post-cutover).

No public API change; Continuation/Output/Input shapes are unchanged. A side benefit: prior tool-call index is now preserved across turns (the dict round-trip silently dropped it).

Related issue

None

Test plan

• just check - green (415 passed, 19 deselected; ruff format/lint + mypy clean).
• Updated tests/interaction/test_output.py for the new completion_status default (weird_unknown → clean).
• Added tests/providers/test_local_contract.py::test_local_reasoning_is_display_only_and_not_replayed, proving local-server reasoning_content surfaces on Output.reasoning but never enters the continuation's replay messages/provider_state or the next replayed payload (the sketch's Local Providers rule).
• Continuation assembly itself is covered by the existing interaction/pipeline boundary tests, which all pass against the typed builder.

Notes

• Breaking internal only - pollux.continuation and pollux.interaction.adapters were never part of the public surface (not in pollux.__all__), and no code outside src/ referenced them.

[codecov]

Codecov Report

❌ Patch coverage is 90.24390% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/pollux/interaction/continuation.py	91.30%	2 Missing ⚠️
src/pollux/parts.py	84.61%	2 Missing ⚠️

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