refactor(object_store): lazy fetch returns PayloadView

[pabloinigoblasco]

Summary

Change the lazy ObjectStore payload contract so producers publish bytes through a sdk::PayloadView (Span<const uint8_t> + BufferAnchor), and the store preserves the anchor end-to-end. No bytes copied across the lazy path; no assumption on the anchor's concrete type.

pushLazy signature

ObjectStore::pushLazy now takes std::function<sdk::PayloadView()> (alias LazyCallback) instead of std::function<std::vector<uint8_t>()>. Producers that already hold bytes behind a shared_ptr — a chunk cache, an mmap region, a parquet column slice, any shared_ptr<T> — capture it in the closure and return a view backed by it. Zero copy.

Type-erased anchor preserved

ResolvedObjectEntry, ObjectBytesBox, and the lazy variant alternative all carry a sdk::PayloadView. resolveEntry propagates the anchor verbatim — no static_pointer_cast, no anchor-type assumption, no Span-vs-anchor mismatch. Producers anchor on any shared_ptr<T>; consumers read through the Span unchanged.

ObjectEntry::payload typed variant

using SharedBuffer = std::shared_ptr<const std::vector<uint8_t>>;
using LazyCallback = std::function<sdk::PayloadView()>;
std::variant<SharedBuffer, LazyCallback> payload;

Two named aliases for the two payload shapes. Compile-time exhaustive dispatch via std::get_if.

Convenience helper

sdk::makePayloadView(std::vector<uint8_t>) in pj_base/buffer_anchor.hpp: wraps a fresh vector as both anchor and Span. For producers with no upstream anchor (e.g. raw bytes from a C-ABI fetch); when an upstream allocation already exists, construct PayloadView directly to skip the helper's copy.

Minor consistency cleanups

• pushOwned / pushLazy return Status (alias for Expected<void, std::string>); the one other site in service_registry_builder.hpp was switched too.
• PayloadView(std::shared_ptr<std::vector<uint8_t>>) constructor takes shared_ptr<const vector> to match SharedBuffer and makePayloadView. Implicit shared_ptr<T> → shared_ptr<const T> conversion makes this non-breaking.

API impact

Source-incompatible for direct consumers of:

• ObjectEntry::payload — switch from std::get_if<shared_ptr<...>> / std::get_if<function<vector<...>()>> to std::get_if<SharedBuffer> / std::get_if<LazyCallback>.
• pushLazy's fetch signature — closures now return sdk::PayloadView. Use sdk::makePayloadView(vec) for the trivial vector case.
• ResolvedObjectEntry — was {Timestamp, shared_ptr<const vector<uint8_t>> data}, now {Timestamp, sdk::PayloadView payload}. Bytes via entry->payload.bytes.data() / .size(); anchor via entry->payload.anchor.

pushOwned is unchanged.

Tests

./build.sh --debug && ./test.sh → 62/62 passing under ASAN.

Two regression tests added in pj_datastore/tests/object_store_test.cpp:

• PushLazyPreservesAnchorType — pushes a shared_ptr<TestBuffer> anchor (not a vector). Resolution must preserve type erasure; a static_pointer_cast<const vector> would UB here.
• PushLazyHonorsSpanSubview — anchor holds 100 bytes; Span covers [20, 30). Resolution must propagate the Span verbatim, not the anchor's full extent.

Version bump

Source-incompatible SDK change in a public header (pj_datastore/object_store.hpp), so per the pre-1.0 versioning convention this warrants a MINOR bump 0.4.2 → 0.5.0, bumped in this PR.

Downstream

• pj4 consumers updated in parallel for the new ResolvedObjectEntry shape and the LazyCallback-returning pushLazy.