Build a scenario's package (mesh + inputs) after showing what it will cost, and poll until built.

Order of operations, so the number is shown before anything is spent:

1. GET the scenario detail. A record with no boundary/computed_status keys is a non-member's read of a public project → refused: a build needs the EDITOR role.

2. Re-call checks on latest_run (a re-POST is NOT deduplicated after built/complete — it would dispatch a duplicate build): no run → proceed; run created/building → resume polling, no POST; run built/queued/computing/processing/complete with latest_run_is_valid not false → return that state, no POST unless rebuild=true; run error/cancelled, or latest_run_is_valid false (the scenario was edited since the build) → proceed.

3. Spend gates, each a refusal with outcome: "refused" and no POST: the scenario has no boundary or its boundary row has no features (the server would admit the build and fail it in make_package); the estimate is None (resolution is 0/unset); the estimate is above 100,000 triangles and confirm is not true — the refusal states the number; re-call with confirm=true to proceed. An estimate of 0 over a boundary WITH features is buildable and is reported, not refused.

4. POST /projects//scenarios//build/. Every 4xx comes back verbatim as a normal result with http_status: 422 MESH_TOO_LARGE (error_code, estimate, ceiling, detail — the server's hard ceiling; coarsen resolution or shrink the boundary; no run was created), 409 COMPUTE_TARGET_UNAVAILABLE, 400/403/404. A 409 WITHOUT an error_code is the dedup body ({status, run_id, detail}: a build is already in flight) — the tool resumes polling that run.

5. On 202 poll the detail's computed_status (created → building → built | error), bounded by timeout_seconds.

Returns a COMPACT state, never the whole detail: outcome (built, error, cancelled, complete/queued/computing/processing for a run past the build, timed_out, or refused), computed_status, mesh_triangle_count_estimate, posted, the POST's build body + http_status when one was made, and the latest run's run_id, run_status, error_message, user_message, mesh_triangle_count. timed_out is normal (make_package re-downloads the terrain from S3 every build; minutes): call again with the same arguments — step 2 resumes polling the same run, it never POSTs twice — or cancel_run(run_id) if the run is stuck in created with no worker. error is terminal: error_message says why (fix the inputs, then call again — an errored latest run is rebuilt).

Units: resolution — on the SCENARIO and on every MeshRegion FEATURE — is a LENGTH in metres; ANUGA maximum_triangle_area = resolution²/2 (FloatField default 100, not nullable; 0 makes the estimate None), and the estimate prices both the same way (TASK-3186). The smallest value becomes the raster cell size. A MeshRegion feature must carry resolution_units = m (features drawn in the map are marked automatically); an unmarked one makes the build answer 422 MESH_REGION_UNITS_UNMARKED, naming the conversion an operator runs.