Slice 7: Deprecate .inner + explicit as_upstream_object_store() escape hatch

[wjones127]

Parent PRD

#7331

What to build

Mark ObjectStore::inner #[deprecated] once common internal call sites have Lance-method alternatives, and add an explicitly named upstream-interop accessor (e.g. as_upstream_object_store()) for the remaining legitimate compatibility uses. The field stays public until the next breaking API window (Phase 6, deferred).

Deprecation note should point users to Lance methods or the explicit escape hatch:

#[deprecated(
    note = "use Lance ObjectStore methods instead; direct upstream object_store access bypasses Lance I/O wrappers"
)]
pub inner: Arc<dyn object_store::ObjectStore>,

Acceptance criteria

• inner annotated #[deprecated] with a message pointing to Lance methods / the escape hatch
• as_upstream_object_store() (or agreed name per Open Q3) added for explicit interop
• Workspace builds with no internal #[deprecated] warnings (remaining uses go through the escape hatch and are documented as compatibility)
• Parent .inner tracker reflects the final set of intentional remaining uses
• Open Question 3 resolved and recorded

Blocked by

• Blocked by Slice 5: Migrate read .inner call sites to ObjectStore methods #7334
• Blocked by Slice 6: Migrate mutation .inner call sites to ObjectStore methods #7335

Design sections addressed

• Phase 5 (Deprecate .inner); Open Question 3