btc(stratum): integer-exact unconditional v35 finder fee in mined-block coinbase (D2)

[frstrtr]

D2 — integer-exact, unconditional v35 finder fee in the mined-block coinbase

Closes the June-audit D2 residual. The audit cited coin/work_source.cpp ~421-433; the actual live code is src/impl/btc/stratum/work_source.cpp (path moved), where the block-coinbase finder fee still computed as float coinbasevalue/200.0 and applied all-or-nothing (skipped entirely when the donation output could not cover the full fee).

Both diverge from the p2pool reference — data.py generate_transaction, verified against ~/p2pool-jtoomim@ece15b0:

amounts[finder]   += subsidy // 200          # 0.5%, integer floor, unconditional
amounts[donation] += subsidy - sum(amounts)  # residual balances to subsidy

The consensus share gentx was already integer-exact; this is only the block-coinbase path, but it is a latent satoshi-rounding divergence on the money path of a won block.

Change

• New SSOT src/impl/btc/stratum/finder_fee.hpp — v35_finder_fee_split(coinbasevalue, donation_sats) = floor(coinbasevalue/200) (integer, subsidy//200), capped at the donation available so the coinbase stays balanced (total == subsidy) and no output goes negative. No float in the split.
• work_source.cpp calls the SSOT; fee is now applied unconditionally (never all-or-nothing skipped — the cap only binds in a pathological rounding edge).
• Base stays coinbasevalue, matching core/web_server.cpp:1888 (in p2pool subsidy = template coinbasevalue, incl. fees).

Test

finder_fee_integer_exact_test.cpp, compiled into the existing allowlisted btc_share_test target. Pins the split vs the subsidy//200 formula at round + odd coinbase values, the sub-200 zero edge, the donation cap, and a float-vs-integer divergence witness (at cbv=312500199 the old float rounds to a different satoshi than the integer floor).

Results: D2 KAT 4/4 green; full btc_share_test 50/50 (no regression).

Review -> integrator. No self-merge.