Add optional short alliance victory

docs/OPTIONAL_ALLIANCE_VICTORY.md

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+# Optional three-stronghold alliance victory
+
+**Prototyped / Partial — 28 October 2026.** The host can select a shorter
+two-member alliance game in the lobby. With at least three physical
+controllers, allied factions need three distinct qualifying strongholds
+instead of the default four. A solo faction keeps its normal target, a
+separately agreed extra-stronghold requirement is added afterward, and Tech
+Tokens retain their existing ownership requirements. The same calculation
+drives public progress and the Mentat victory decision. Changing the setting
+clears human readiness; older games omit it and retain the default.
+
+The 2019 GF9 rulebook's optional alliances table, printed p. 24, lists a
+two-member alliance with three strongholds as a shorter game. The
+[publisher rulebook mirror](https://www.boardgamesbot.com/assets/games-library/dune/manual_eng.pdf)
+preserves that table. This first version keeps the existing four-stronghold
+target for two physical controllers, including the two-faction-per-person
+setup; priority between that special setup and this optional table is a
+provisional interpretation. Larger alliance variants require separate
+membership and interaction work. Advanced and expansion combinations are not
+certified by the Basic-focused evidence.