feat: native append/replace/upsert load modes

[shefeek-jinnah]

🚧 Draft — depends on the SDK's mode/key support (hotdata-framework, separate PR) and a regenerated hotdata client whose managed-table decls carry key. Dependency floors are bumped with TODO(release) markers; un-draft once those are released and the exact pins are set.

What

Push loads down as the server's native load modes instead of a client-side read-modify-write.

• append / replace upload the batch directly — no full-table read + rewrite.
• merge → native upsert by the resource's primary_key (updates matches, inserts the rest); the table's key is declared up front (initialize_storage + reconcile-add).
• insert-only, and a merge with no resolvable primary key, still combine client-side and replace (the _dlt_id fallback can't match a logical row across runs).

Also fixes two combine-path bugs

• resolve_primary_key now reads dlt's per-column primary_key hints — it read a top-level table key dlt never sets, so merge silently deduped by the row-unique _dlt_id and produced duplicates across runs instead of updating in place.
• Arrow string_view/large reads are normalized to plain string/binary before combining (fixes ArrowTypeError: string_view vs string).

Testing

• 86 passed, 2 skipped offline; the in-memory harness now models the native modes, so test_merge_dedup validates the native upsert path.
• Server side proven on a live deploy (native append/replace/delete/update/upsert via the HTTP API).
• Pending: native upsert/delete through the live connector — needs the regenerated client.

Before un-drafting

• Set exact dep pins (hotdata, hotdata-framework).
• Run ruff + mypy.