Explorer node: add asset_swaps and asset_swaps_totals endpoints

[Maxnflaxl]

Summary

Implements #2053. The explorer node already exposes atomic-swap offers via swap_offers / swap_totals, but had no equivalent for Asset Swaps (DEX). This adds two read-only endpoints:

• /asset_swaps — live asset-swap (DEX) offers
• /asset_swaps_totals — per-asset aggregate totals + offer count

The node already receives this data over SBBS broadcast, so no wallet / wallet-API is required.

Both endpoints are documented in the Beam Node Explorer API reference.

How it works

The adapter builds a read-only DexBoard over the broadcast router and wallet DB it already creates for the atomic-swap board. It subscribes to the DEX offers broadcast channel and accumulates offers (foreign offers are kept with isMine=false, not rejected). A no-op IRawCommGateway is passed because the explorer only lists offers — it never accepts or processes trades.

Plumbing mirrors the existing swap_offers / swap_totals path exactly: an entry in the ExplorerNodeDirs macro, OnRequest handlers, and backend methods. The new code is gated by BEAM_ASSET_SWAP_SUPPORT, matching how atomic swaps are gated by BEAM_ATOMIC_SWAP_SUPPORT (required, since the wallet DB's DEX methods are themselves behind that flag).

/asset_swaps

Reported in maker terms (send_* = what the maker offers, receive_* = what the maker wants), using the absolute first/second sides rather than DexOrder's isMine-relative send/receive helpers (which are perspective-dependent and meaningless on a neutral node). Amounts are formatted with PrintableAmount, timestamps returned as raw unix seconds (formatted client-side, as on the block-headers page).

{
  "id": "<order-uuid>",
  "send_asset_id": 0, "send_currency": "BEAM", "send_amount": "4,862.63068319",
  "receive_asset_id": 7, "receive_currency": "USDT", "receive_amount": "500",
  "create_time": 1781784005, "expire_time": 1781785805
}

/asset_swaps_totals

{
  "total_offers_count": 12,
  "assets": [
    { "asset_id": 0, "currency": "BEAM", "amount_offered": "5,000", "amount_wanted": "0" },
    { "asset_id": 7, "currency": "USDT", "amount_offered": "0", "amount_wanted": "2,500" }
  ]
}

Bugfixes

Fixes the explorer needed alongside the feature:

• PoW block timestamps rendered as 1970-01-01. get_Timestamp_s() and the hdrs Age / block-duration deltas read SystemState::get_Timestamp_ms(), which carries a value only for PBFT/PoS consensus — on a PoW chain it reinterprets the PoW solution as PBFT header data and returns 0, so every block / header / status timestamp (and the Age and d.Time columns) collapsed to the epoch. Regression from 221cc7da2 ("explorer: showing time in ms where applicable (for PoS networks)"). Fixed by choosing the source from the chain's consensus type (Rules::m_Consensus) via a single get_Timestamp_ms_safe() helper used at every affected site: the ms timestamp for Pbft, the seconds field m_TimeStamp for PoW / FakePoW.

• -Wdeprecated-literal-operator build break. Recent clang flags the whitespace-before-suffix user-defined-literal spelling in the bundled 3rdparty/nlohmann/json.hpp; with the project's unconditional -Werror this breaks the build of any target that includes it (e.g. explorer-node). Added -Wno-deprecated-literal-operator to the AppleClang flag block, next to the existing -Wno-enum-constexpr-conversion handling.