dash(S8): get_work() consumer capstone — fuse NodeCoinState::select_work() + assemble_work_job_targets()

[frstrtr]

S8 get_work() consumer capstone — the first production caller of the embedded arm

Stacks on #694 (reception-wire leg 4). Base = dash/s8-reception-wire-mnlist-leg so the diff is this one commit; rebases clean to master once #693 → #694 land.

What was missing

The embedded get_work chain was fully wired but had no production consumer. Both halves existed and were unit-tested in isolation:

• NodeCoinState::select_work() (dash(S8): embedded_gbt live-wire selector + retained dashd fallback #672/dash(S8): node-held coin-state flips select_dash_work hot arm (stacks on #672) #673) — the template source: embedded build_embedded_workdata() (oracle parity vs frstrtr/p2pool-dash getwork(), older-than-v35) when the node-held bundle is populated by the 4-leg reception wire (dash(S8): reception-wire legs 1+3 -- new_tx/block_connected -> CoinStateMaintainer (rebased on master) #693/dash(S8): reception-wire leg 4 -- mn_list_update -> on_mn_list_update (closes the 4-leg wire) #694), else the retained dashd getblocktemplate fallback.
• assemble_work_job_targets() — the per-miner job-target arithmetic (work.py:368–426).

Nothing fused them. Only the test suites ever called select_work().

This slice

dash::stratum::get_work() (src/impl/dash/stratum/get_work.hpp) — the single miner-facing entry, the first non-test caller of the embedded arm:

• sources the base template off the node-held NodeCoinState — no direct dashd poll on the hot path when the bundle is populated;
• assembles the job targets over whichever template arm ran;
• dashd_fallback stays REQUIRED, invoked only on a set-gap (unpopulated / invalidated bundle) — the always-reachable safety + [GBT-XCHECK] cross-check arm, never removed.

Evidence

test_dash_get_work — 4/4 green (seeding mirrored exactly from test_dash_node_embedded_wire, no fabricated oracle values):

1. set-gap → DashdFallback (fallback invoked once) and targets still assembled;
2. populated → Embedded, fallback not invoked, byte-equal to a direct build_embedded_workdata() over the node's own held state;
3. job targets identical across both template sources;
4. on_invalidate() (reorg) demotes back to the fallback.

Local: dash object lib builds rc=0; ctest -R ^DashGetWork → 4/4 (out of 4). Registered in both build.yml --target allowlists.

Safety posture

STRICTLY single-coin (src/impl/dash/ + test only), SAFE-ADDITIVE — no bitcoin_family / src/core / other-coin reach, per-coin isolation intact. dashd RPC fallback preserved. No self-merge — integrator verify (oracle + full CI) → operator tap.