Path math and indexing unit tests for grid/path materializer

[jasonappah]

Objective: Add high-confidence tests for grid/index math and path materialization to prevent off-by-one and orientation bugs.

Context:

• Target src/server/robot/grid-indices.ts, src/server/robot/path-materializer.ts, and src/common/spline.ts.
• Include corner/castle/edge cases and invalid inputs.

Acceptance:

• New tests (e.g., under src/server/robot/test/) cover grid conversions, path generation length/shape, and boundary conditions.
• Tests run in CI and pass deterministically.
• Bugs found during test writing are fixed with clear commits.