explorer: restore treasury view by treating explicit height=0 distinctly from a missing height

[Maxnflaxl]

Summary

The treasury view (?network=mainnet&type=treasury, which requests block?height=0) was silently broken: it displayed the current tip block instead of the treasury UTXO groups.

The cause is #1990 ("display current block instead of treasury when calling block with no arguments"). That change implemented "no arguments" as if (height) { … } else { return tip }, which treats an absent height and an explicit height=0 identically. As a result, the only route that reaches get_treasury() on a synced full node was removed, and the frontend's treasury link — which deliberately sends height=0 — started rendering the latest block.

The get_treasury() fallback in get_block() was noted at the time as "kept for safety … in practice never reached under normal operation." That was accurate, but it was the symptom: the code path had a real caller (type=treasury → height=0), so making it unreachable broke a feature rather than pruning dead code.

Root cause

"No argument" and "height=0" were never actually distinct in this code — the ambiguity is structural, in two layers:

1. Height is uint64_t and 0 is a valid height (the treasury pseudo-block), yet 0 is also the "unspecified" sentinel. The value meaning "give me the zero-height block" is the same value meaning "caller gave nothing."
2. get_int_arg("height", 0) erases the distinction that still existed in the raw request. The args map knows whether height was in the URL, but get_int_arg returns the default 0 for an absent arg — the identical int64_t produced by an explicit &height=0. From that call onward the two are physically indistinguishable, and get_block's if (height) reads both as "no height."

So it wasn't a deliberate decision to equate them — the code simply never carried a "was it provided?" bit, and reused 0 for two meanings. That's why the fix has to reach underneath get_int_arg.

Fix

Distinguish the two cases at the router level, in Server::on_request_block, by checking the raw args map (the one layer that still preserves presence-vs-absence):

• height absent → latest block (preserves Explorer node: Display current block instead of Treasury when using "block" with no arguments #1990's intent; bare block still shows the tip)
• explicit height=0 → treasury (restores the existing frontend link, no HTML change)

A get_treasury() method is added to the IAdapter interface so the router can dispatch to it directly, and the existing fallback inside get_block() is annotated as a genuine edge-case guard (fresh node with no blocks, or prune-navigation via adj<0).

Changes

• explorer/server.cpp — in on_request_block, route an explicit height=0 to get_treasury(); a missing height still falls through to get_block().
• explorer/adapter.h — add virtual json get_treasury() = 0; to IAdapter.
• explorer/adapter.cpp — expose get_treasury() (wraps the existing private get_treasury(json&)); document the in-get_block fallback as an edge-case guard.

Behavior

Request	Before	After
block (no height)	tip	tip
block?height=0	tip ❌	treasury ✅
block?height=N	block N	block N
?type=treasury (frontend)	tip block ❌	treasury UTXO groups ✅

No frontend change required. Compiles clean (explorer target).

Testing

Tested the above behaviors, all good.