feat: round 3 — LRU cache eviction, OHLCV validation, silent exception logging, Order UUID, slippage cap, BB/SR rendering, multi-stock switcher

[Copilot]

Third optimization round addressing unbounded memory growth in the data cache, silent error swallowing, order ID collisions under fast iteration, and missing chart overlays in the report viewer.

Data layer

• LRU cache: _cache replaced with OrderedDict + eviction at 200 entries to prevent unbounded growth in walk-forward / paper trading
• OHLCV sanity check: _validate_ohlcv() rejects frames missing required columns, non-positive close, or high < low — skips to next source on failure
• Calendar boundary warning: _warn_calendar_coverage() emits UserWarning when end_date.year > 2028 (hardcoded holiday list limit)
• Exception logging: All 30 except Exception: pass → except Exception as e: log.debug(...) — control flow unchanged, failures now observable

Backtest reliability

• Order ID collision: datetime.now().strftime(...) → uuid.uuid4().hex[:12]
• VolumeSlippage cap: impact_pct now clamped to max_slippage_pct (default 5%) to prevent runaway slippage at low volume:

  VolumeSlippage(impact=0.05, max_slippage_pct=0.05)
  # impact_pct = min(impact * (amount / daily_volume), max_slippage_pct)

• Docstring: simple_factor_analysis() now explicitly states it is not a Fama-French model

Report viewer (ReportViewer.tsx)

• Bollinger Bands / Support-Resistance: Renders bb_upper/middle/lower_data (dashed) and support/resistance_data (dotted) on the K-line chart — type fields already existed but were unused
• Multi-stock switcher: When symbols_list has >1 entry, renders a <select> dropdown that merges symbols_data[symbol] into the effective chart data via selectedSymbol state

All 391 existing tests pass, 0 CodeQL alerts.