Read a player's lobby slot from the scenario for Slot Of #99

Problem

After #97, Bastion on athena main (docs/scenario-examples/valid/bastion.json plus buttons for every slot on both players) ends not_modeled at tick 0:

{ "status": "not_modeled", "construct": "value getSlot", "tick": 0,
  "location": { "rule": 168, "name": "[Event/先知] Attach prophet next-event text", "eventPlayer": "p2", "action": 0 } }

Bastion calls Slot Of(Event Player) / Slot Of(Victim) in many places (e.g. staggering Wait(Multiply(Slot Of(Event Player), 0.032) ...)).

Design

A player's lobby slot is a scenario fact the simulator cannot derive; roster order is not a measured slot. Follow the invariant for absent inputs: add an optional player field slot (integer, 0..=11) in the scenario format. Slot Of(player) reads it; when the player has no slot, the run ends missing_input with input name slot for that player (no default from roster order). An out-of-range or non-integer slot is rejected at load. Two players with the same slot are rejected at load.

Scope

• scenario.rs: parse and validate slot; eval/call.rs: implement getSlot for a player operand; non-player operand stays not_modeled.
• Docs: docs/scenario-format.md (field table, modeled-reads list, missing-input list), docs/simulator-interface.md if it lists reads.
• Leave the Slot N rule filter as is (still not_modeled) unless it is a small change to match on the declared slot; if so, do it and document it, otherwise note it in the PR.
• Add slot to docs/scenario-examples/valid/bastion.json players (distinct values), plus a v1 example test if examples are tested.

Acceptance

• Tests: slot present yields the number; absent yields missing_input naming slot; duplicate and out-of-range slots rejected; non-player operand not_modeled.
• Bastion rerun with buttons for every slot: report the new first stop, or completion. Parent: Run Bastion to completion on athena-sim #45.