Reject unconsumed options in explicit solve mode

[ocots]

Summary

• Reject unconsumed keyword arguments in explicit solve mode instead of silently ignoring them.
• Provide actionable errors for strategy-specific, ambiguous, and unknown options.
• Clarify in the explicit-mode documentation that strategy options must be configured at construction time.
• Add regression coverage for backend, unknown options, dispatcher validation, and bypass.

Test plan

• Targeted explicit solve tests: 28 passed.
• Complete solve test suite: 562 passed.
• Full package test suite: 2,184 passed; 2 expected optional-capability tests marked broken.
• Documentation build: blocked by existing CTLie and extension precompilation errors unrelated to this change.

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