feat(budget): add simulation, projection and policy methods

[codexhange]

Adds BudgetResource.simulate, project and validation helpers plus the BudgetSimulationRequest types backing them.

Closes #117

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[mergekeeper]

MergeKeeper review

Scope: in scope for linked issue #117.
Verdict: clean

Successfully implements budget simulation and policy check methods with full type definitions and unit tests matching all acceptance criteria.

Reviewed commit: e976d1ca2cc4a21dda9e3d2bf8fe8386fcae378c.
CI and merge eligibility are checked separately.

[mergekeeper]

MergeKeeper merge status

Status: blocked
PR state: open
Mergeability: conflicts / not mergeable
Checked commit: e976d1ca2cc4a21dda9e3d2bf8fe8386fcae378c.

Reason: One or more required CI checks failed.

Failing checks:

• build - view details

Next steps:

1. Open the failing check details above and fix the reported error.
2. Run the same checks locally where possible.
3. Commit and push the fix.
4. MergeKeeper will automatically re-review the updated PR.