docs(dash): coordinated v36-universal migration plan — dual-stack + flag cutover (block-viable gated)

[frstrtr]

DASH lane owed deliverable (operator [s=migration-plan]). Docs-only, SAFE-ADDITIVE, single new file docs/dash/v36-coordinated-migration-plan.md — no coin tree, no consensus, no CI-config.

Block-viable HARD precondition now SATISFIED on master (plan gate items 1-3 all true):

1. dash(s8): broadcaster_full won-block keystone — dual-arm relay (#83) [fresh base, supersedes #456] #465 broadcaster_full keystone MERGED (35693c5) — won-block dual-arm relay (embedded P2P + dashd submitblock fallback).
2. G3a POPULATED regtest block proven (diverse tx types, submitblock arm) + G3b populated testnet3 leg proven (block h120 nTx=4, accepted+relayed).
3. dashd RPC fallback retained on every node — never removed.

Plan substance: DASH runs a coordinated cutover (short dual-stack window -> scheduled flag moment), NOT a slow 1-by-1 ratchet like DGB/BTC/LTC, because the friendly DASH p2pool node set is enumerable + operator-coordinated (operator directive 2026-06-18). Transition 16->36 (older-than-v35 baseline vs oracle frstrtr/p2pool-dash @9a0a609). Preserves the 3-bucket isolation invariant (PREFIX/IDENTIFIER untouched; FLAG6 donation carried; v16 compat temporary, dropped at cutover+grace). Phases M0 enumerate/pin -> M1 dual-stack 72h -> M2 coordinated cutover -> M3 grace + compat drop.

GPG-signed (50AB1379285EFE76). No self-merge: integrator verify -> operator tap.

[frstrtr]

Withdrawing: opened prematurely off the stale Jun-25 public branch (83-line stub). The finalized coordinated-migration plan (341 lines: crossing-floor, #536 threshold-SSOT, work-weighted metric, window-accessor, PPLNS two-function, vardiff G1 parity, S8 binding spec, per-phase GO/NO-GO gates, M2 partial-flip abort, G3b evidence-of-record) is held-for-operator-push per operator UID3282, not for an autonomous public PR. No content lost; deliverable stands as done+held. Sorry for the noise.