feat(sdk): add scenario evaluation and flat-rate spot solving #45

Add Python SDK support for the scenario API in API MR !209.

• Sync/async evaluate_scenarios and solve_spot, typed requests/results, quote support and explicit CU ceilings.
• Preserve ordered partial scenario errors, all point/interval boundaries, unresolved coverage, exact diagnostics/provenance and known/unknown charge receipts. No local pricing/solving, hidden requests or automatic retries.
• Consume the immutable API contract at 7b4f233b2222fbb95b2492d5c57dbde1ac32e571; document the final flat solve rates: 100 CU BSM, 200 CU BS1993, with no public internal pricing formula.
• Executable synthetic examples and standalone scenario/solver notebooks with inert setup, explicit actions and current-quote checks.
• Contract, transport, failure, native integration, Python 3.11/3.14, installed-wheel and Docker notebook validation.

Owner hold: do not merge the SDK PR into main or release until the matching API has been deployed. Before lifting the hold, verify the live API schema against the pinned contract, scenario permissions, authenticated solve quotes and 100/200-CU receipts. Keep the PR draft with auto-merge off. This issue does not authorize API deployment or SDK publication.

Related: SDK #2, #6, #7; API #34 and #146.

Solver acceptance app

• Add examples/solver_app, a runnable local browser app using the SDK against the API. Both models, both targets, explicit quote/cap/solve flow, full coverage/boundary displays and session receipts are required.
• App transport tests and protected native candidate API checks pass, including 100/200-CU receipts and incomplete/non-unique outcomes.
• Complete and retain the browser walkthrough. Computer-use URL policy blocked visual verification; do not treat API-level app checks as browser sign-off.
• Verify the matching deployed API before merging or releasing SDK PR feat(sdk): add scenario evaluation and spot-target solving #46.

This app supplies the solver-client acceptance witness for API #34 and #146. It does not introduce local pricing formulas or a local root finder.