[PPTX] Slides 1-2: Cover slide + Executive Summary slide implementation #42

Part of epic: #32
Depends on: #41 (core module skeleton must exist first)

Slide 1 — Cover

Full dark-blue (GS_BLUE) background. Elements:

• Company name in large white bold text (centre)
• Ticker + Exchange in smaller white text below
• Rating pill: coloured rectangle (BUY=green, HOLD=grey, SELL=red) with white text
• Target price: TP: $XXX right-aligned
• Date + "Sellside Research Engine" watermark in footer
• Gold horizontal rule separating company from metadata

Slide 2 — Executive Summary

2x3 grid of 6 key fields (from issue #21):

• Rating | Target Price | Variant View
• Top Thesis Point | Top Risk | Next Catalyst

Each cell has: field label in small GS_BLUE text, value in larger black bold text. Rating cell background is colour-coded.

Acceptance Criteria

• _add_cover_slide() implemented with all elements listed above
• _add_exec_summary_slide() implemented with 2x3 grid
• All text is editable in PowerPoint (not images)
• Cover slide background is full-bleed GS_BLUE
• Rating pill colour matches BUY/HOLD/SELL
• Exec summary renders cleanly at 1920x1080 when exported as image
• Unit test: test_pptx_cover.py asserts slide count >= 2 after build()