Remove the saved-view source operations

[callumalpass]

Saved views are records claimed through mdbase.view (#56) or obsidian.base (#57), so the four saved-view source operations duplicate the record operations.

• Removes read_view_source, create_view_source, update_view_source and delete_view_source, and the writable_view_sources conformance feature.
• The replacement is read with include_document, update with document (written exactly as supplied), create and delete, under their ordinary validation, path and concurrency rules.
• list_views reports source.writable: true for sources that are records.
• The Bases adapter keeps configured x-obsidian.bases.include discovery, read-only (writable: false) and transitional, together with view_changed for those non-record sources. They go once collections have adopted Bases as records. The adapter previously tied that removal to this change; it is deferred so collections that have not yet approved base record extensions keep their views.

No conformance cases referenced the removed operations. check_test_levels and check_v03_tests pass.

[callumalpass]

Holding: hosted collections store Bases and mdbase views as view resources, so on hosted collections the view-source operations are still the only write path. This waits until the hosted provider treats view sources as records.