build_obstructions_array() silently erases manually added visual obstructions

`build_obstructions_array()` rebuilds `obstructions_array` from `obstructions_collection` and assigns over the old array, so any walls added earlier via `add_visual_obstruction()` are discarded without warning. The implicit contract — build first, add after — holds in the `read_fds_data` flow (which builds last thing) and is now noted in `set_grid`'s docstring on #41, but the method itself neither documents nor preserves manual walls; for a synthetic scene (`obstructions_collection` empty) a stray call wipes everything back to zeros.

Cheapest: a docstring warning on `build_obstructions_array` itself. Better: merge instead of overwrite (`|=` onto the freshly built array), which preserves manual walls in both flows and cannot break existing callers that follow the contract. Found while reviewing #41; pre-existing behaviour, so filed rather than folded in.

Start by locating build_obstructions_array() and reading how read_fds_data and set_grid use it, alongside add_visual_obstruction(). Confirm the current rebuild discards manually added walls, then preserve those walls or document the contract as appropriate; verify that synthetic scenes and the existing read_fds_data flow retain their expected obstructions.

Written by the indexing model from the issue text.