feat(core): harden v2 interaction contracts

[seanbrar]

Summary

Hardens the v2 RC boundaries required by downstream agent runtimes: public provider-aware environment identity, opaque schema-v2 continuation state with validated portable history messages, and deterministic cancellation/early-stream cleanup through core and every provider adapter.

Related issue

None

Test plan

• just check — Ruff formatting/lint, Markdown lint, mypy, and 472 non-API tests passed; 19 credentialed API tests were deselected by the standard check target.
• just docs-build — MkDocs built the documentation successfully.
• Boundary coverage includes environment fingerprint semantics, continuation schema/provider compatibility, portable text and parallel tool history, Anthropic signed-thinking replay, one-shot/session cancellation, and explicit early stream closure across all provider adapters.

Notes

This intentionally uses the RC window for breaking corrections. Schema-v1 RC continuations are rejected; Continuation construction/state inspection and transcript conversion helpers are removed; EnvironmentSnapshot is no longer public; and system history moves to Environment.instructions. Runtime policy, persistence, tool execution, memory, and idempotency remain downstream concerns.

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• PR title follows conventional commits
• just check passes
• Tests cover the meaningful cases, not just the happy path
• Docs updated (if this changes public API or user-facing behavior)

[codecov]

Codecov Report

❌ Patch coverage is 85.64356% with 29 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/pollux/_lifecycle.py	63.63%	12 Missing ⚠️
src/pollux/interaction/continuation.py	90.00%	12 Missing ⚠️
src/pollux/interaction/environment.py	58.33%	5 Missing ⚠️

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