Add first-class strategies API with standardized signal contracts

[TDamiao]

Goal

Promote strategy playbooks from examples into a supported API module with consistent signal contracts.

Why

Examples exist, but production users need reusable/parameterized strategy primitives.

Implementation prompt

Add strategies/ exports for baseline strategies:

• MA cross
• RSI mean-reversion
• BB squeeze
• Donchian breakout

Common output contract:

• signal: -1 | 0 | 1
• confidence?
• meta (inputs used, warmup status)

APIs should accept both batch arrays and candle-based inputs where applicable.

Scope

• Pure strategy signal generation (no execution/backtest side effects)
• Config objects with defaults and strict validation
• Shared helper utilities for warmup and null-safe behavior

Acceptance criteria

• At least 4 strategy builders available via ta-crypto/strategies
• Consistent typed return contract across all strategies
• README strategy section with usage examples
• Unit tests for warmup, parameter bounds, and deterministic outputs

Test plan

• Golden fixtures for each strategy baseline
• Cross-check one strategy against an external notebook/reference script
• Smoke tests combining strategy -> backtest integration (when backtest module exists)

[TDamiao]

Tracked by v0.4 master gate: #21.