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

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 #7334
• Blocked by #7335

Design sections addressed

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

Start with parent issue #7331 and the blocked work in #7334 and #7335, then locate ObjectStore::inner and its internal call sites. Done means the field is deprecated, explicit upstream interop is available, internal warnings are gone, intentional uses are documented in the parent tracker, and Open Question 3 is recorded as resolved.