[fm] Add disk diagnoser with typed fact tables

[smklein]

This PR provides an alternative to #10460 , using strongly-typed "enum-ish" tables for each DE's set of facts, rather than a JSON blob. It should be mutually exclusive with that PR (we either do JSON, or we don't).

The first fault management diagnosis engine: opens a case for any non-Online zpool whose backing physical disk is currently in service in the control plane, and closes it on recovery or expungement.

Supporting infrastructure introduced along the way:

• DiagnosisEngineKind::Disk variant (Rust + DB enum)
• fm_case_fact child table for per-engine state (one case has 0..N mmutable facts; stable UUIDs across sitreps; participates in copy-forward + GC like other sitrep child tables)
• CaseBuilder::{add_fact, remove_fact, facts} API
• InServiceDisk nexus-types projection consumed by FM, populated from the existing zpool_list_all_external_batched datastore method with policy filtering done in the background task

[smklein]

This looks a little involved, but it's worth noting: This is only used in tests? So doing some one-time complexity to avoid a need for future updates on each new fact type seems like a nice tradeoff.