docs: account risk and capital management primer mapped to Trader

[rustyeddy]

Part of #409 (milestone: Account Margin Admission).

Problem

docs/account-risk.md (untracked draft) is a useful general primer on account-level risk: vocabulary, layered constraints, market-type rules, invariant tests. But it was written against a different codebase:

• It references RunPortfolio, service/portfolio_runner.go, multi-account.md, ExecutionVenue, and AccountManager, none of which exist here.
• It misdiagnoses the 4.6x SPY backtest. The real cause: the unbounded fixed-fraction sizer, an empty risk engine in cmd/trader/backtest, and a simulator with no margin policy.
• It proposes an API that conflicts with accepted ADRs:
  • float64 values (ADR-004)
  • a single RiskManager that sizes, resizes, and approves (ADR-006 and ADR-029)
  • halt, flatten, and kill switch inside risk rather than live guards (architecture: operational guards are separate from trade risk)

Scope

Rewrite and commit docs/account-risk.md so it:

• keeps the general vocabulary and market-type material;
• states the real diagnosis;
• maps each layer and parameter to what exists today, what milestone feat: enforce account-level margin ratio in backtests #409 (docs: ADR-066 account-level initial-margin admission #410-feat: update equity baselines and sizing guidance for margin admission #417) adds, and what is future work;
• follows Trader's ADRs: exact numerics, sizing in Sizer, strict approve/reject risk, live guards separate, and no resizing inside risk;
• links the milestone issues.

Acceptance criteria

• The doc references no code that doesn't exist, except code explicitly labeled as planned or future.
• It is linked from docs: ADR-066 account-level initial-margin admission #410.