MCP: clarify wait()/phase semantics for simultaneous turns

[shaumik]

Status: not started

Why: The current contract is "Wait returns ready when it's your turn
to act" — fine in the abstract, but the engine resolves both sides'
choices simultaneously in PhaseChoosing, and during PhaseReplace
only the side(s) with a fainted active need to act. The view doesn't
distinguish three real cases an agent should care about:

1. You need to choose this turn, opponent also choosing — your call is
   blind to their choice (the common case).
2. You need to choose, opponent doesn't (they're mid-replace from an
   earlier faint, or it's a one-sided replace turn) — your call is
   informed.
3. Opponent needs to replace, you don't — wait() should not return
   ready: true for you, but today it sometimes does with
   phase:"replace", replace:false, which is confusing enough that
   agents have called act and gotten bounced.

An agent that knows it's choosing blind plays more defensively (avoids
fragile-to-prediction switches); an agent that thinks it has full info
will overcommit. This affected play in a real session — a Primeape
switch into a hidden Mewtwo that was decided "blind but the agent didn't
know it was blind."

Scope:

• Add to waitOut: awaiting: "you" | "opponent" | "both" and
  simultaneous: bool. The session already tracks needsAction per
  side via the dispatcher; computing this is cheap.
• wait() returns ready: true only when your action is needed
  (awaiting includes "you"). Pure opponent-replace turns block until
  the opponent acts and the next state frame arrives.
• Document the three cases in
  docs/mcp-protocol.md §3 with the same
  table style as the existing state-machine table.

Minor doc fixes to bundle in:

• foe_bench_alive is the count of unfainted Pokémon on the opponent's
  bench (excluding the active). The field name reads ambiguously
  during faint-replace transitions; either rename to
  foe_bench_unfainted or pin the definition in the protocol doc with
  a worked example. (Don't rename without checking the SPA — web/
  almost certainly reads it.)
• The phrase "Validate against the legal action set implied by the
  latest view" in act's description becomes accurate only once
  MCP: expose the legal-action list on the view #2 ships. Update the description in that PR.

Acceptance: An agent loop that only acts when awaiting includes
"you" never gets errNotYourTurn. Replace-phase turns where only the
opponent is replacing never wake the agent's wait() with
ready: true.

Depends on: nothing.
Sibling: #2 (closely related — both are about
removing "what's actually legal right now" ambiguity).