explorer: fix stale per-element state and ignored adj in /blocks - #2079

Summary

Two bugs in the explorer's /blocks (plural) endpoint, reported by @dbadol as #2077 and #2078. Both live in Adapter::get_blocks, so they're fixed together.

#2077 — every element carries the first block's header

Regression from ba181a9, which replaced the old per-row get_block_impl() walk with a StateID/m_Number loop. The loop advances the row but never reloads the state for it, so every element after the first is rendered against the first block's Block::SystemState::Full:

sid.m_Number.v++;
sid.m_Row = _nodeBackend.FindActiveAtStrict(sid.m_Number);   // `s` left stale

Because the body is read from sid.m_Row (correct) while the header and context come from s (stale), the damage is wider than the report: on top of h and info, the totals table (Chainwork, Current Emission, Current Circulation, Size Compressed, Size Archive) and outputs[].Maturity are all wrong. Verified against mainnet — for block 27730 in a height=27729&n=4 series, emission reads 2,218,320 instead of 2,218,400 and the coinbase maturity 27969 instead of 27970. inputs and kernels were the only correct sections.

Fixed by reloading the state after each step.

#2078 — adj is ignored

adj simply never reached the backend: on_request_blocks parsed only height and n, and get_blocks had no adj parameter. What looked like "defaults to adj=1" was FindBlockByHeight → FindAtivePastHeight snapping to the next active height unconditionally, which also meant /blocks had no not-found path at all.

Fixed per the three points in the issue:

1. adj is parsed and passed through, and applies to the first block of the series only; the rest always follow upwards.
2. A vacant height with no adj returns {"found":false,"height":N}, matching /block.
3. Once the walk passes the tip, the remaining elements are filled with the same not-found objects, so the array always has n entries.

To keep /block and /blocks from drifting apart again, the first-block resolution is now shared: get_block's logic is factored into ResolveBlock(sid, s, h, height, adj) and both endpoints call it.

Changes

• explorer/server.cpp — on_request_blocks parses adj and passes it on.
• explorer/adapter.h — get_blocks(Height, uint64_t, int adj).
• explorer/adapter.cpp — new ResolveBlock helper shared with get_block; get_blocks refreshes the state per element, honors adj for the first block, and emits not-found placeholders.

Behavior

Two judgement calls worth a look, since the spec is ambiguous for PoS chains:

• Point 2 returns a one-element array rather than a bare object, to keep /blocks type-stable for clients. Easy to change if the bare object is preferred.
• The placeholder heights past the tip are consecutive (tip+1, tip+2, …), which only carries meaning on PoW. On a PoS chain with gaps they should be read as "no block here", not as a height prediction.

Closes #2077
Closes #2078