dash(stratum): explicit template-cache invalidate on coin-P2P tip event — close stale-payee window under future io-decouple

[frstrtr]

Fast-follow reward-safety hardening (v0.2.4.1). NOT blocking the current review-approved v0.2.4 hotel deploy — the stale-payee window closes correctly today; this makes it explicit + future-proof.

The latent gap (found in the ZMQ-rebase adversarial delta)

With --coin-p2p-connect (the hotel config), the coin-P2P arm's on_tip_changed handler (main_dash.cpp) does only bump_work_generation() + notify_all(). The fresh-payee GBT re-source happens only implicitly: the coin-P2P arm has no refresh_executor_, so cached_work() takes the legacy inline-blocking re-source path.

If #781's io-decouple is ever extended to the coin-P2P arm, refresh_executor_ becomes set → cached_work() serves the STALE cached template (frozen masternode payee) while refreshing async → a stale-payee job goes out → the window reopens → lost blocks on a rotated payee.

Fix

The coin-P2P on_tip_changed handler now calls ws->invalidate_template_cache() alongside bump_work_generation() + notify_all() — mirroring the #770/#772 fire_refresh trio (invalidate + bump + notify). The invalidate drops the cache, so the next served job is re-sourced with the fresh-tip payee regardless of refresh_executor_ state; a set-gap (honest null) is served until the fresh template lands — never a stale-payee job.

KATs (folded into the allowlisted test_dash_stratum_work_source — no build.yml change)

With a deferred refresh_executor_ (models the rpc_pool background thread) and the fallback rotated to a NEW tip/payee after the cache is primed:

• InvalidateClosesStalePayeeWindowUnderRefreshExecutor — after invalidate+bump the served job is never the stale tip-A template (set-gap, then fresh tip-B). The window stays closed with refresh_executor_ set.
• BumpAloneServesStaleUnderRefreshExecutor — the contrast: bump-only serves the STALE tip-A payee, proving the invalidate is what closes it.

Consensus-neutral — notify/refresh timing only; no share/payout/target/payee-value change. stratum_work_source 40 → 42 green; c2pool-dash builds+links.

Draft — for a v0.2.4.1 hotel binary; the staged v0.2.4 deploys meanwhile.