Add MTF alignment/merge utilities on top of resampling

[TDamiao]

Goal

Add multi-timeframe primitives on top of resampling so users can build top-down strategies (e.g., 1h trend + 5m entry).

Why

Issue #6 covers resampling. This issue focuses on timeframe alignment/merge utilities needed for real MTF strategy logic.

Depends on

• Add candle resampling utilities (crypto timeframes) #6 resampling utilities

Implementation prompt

Implement MTF helpers:

• alignTimeframes(base, higher, options)
• mergeTimeframeFeatures(base, higherFeatures, options)
• Optional forwardFillUntilClose mode with strict boundary controls

Rules:

• No lookahead leakage (only finalized higher-timeframe data)
• Deterministic timestamp boundary handling
• Explicit behavior for gaps and missing candles

Scope

• Typed APIs for alignment/merge outputs
• Strict validation for unsorted timestamps and duplicate bars
• Time boundary options documented (UTC/session-aware)

Acceptance criteria

• No future leakage in merged features
• Alignment works for 1m->5m, 5m->1h, 1h->4h fixtures
• Tests validate gaps/partial higher bar behavior
• README contains one MTF example (trend timeframe + trigger timeframe)

Test plan

• Unit tests with handcrafted timestamp fixtures
• Property-style tests for monotonic timestamps and non-leakage invariants
• Example smoke test using exported API only

[TDamiao]

Tracked by v0.4 master gate: #21. Dependency note: this should land after #6.