Streamed HTML parsing + content sniffing

Summary

Adopt streamed parsing for HTML to reduce allocations, and do early content-type sniffing to skip binary/large content unless configured.

Motivation

• Lower memory usage during large crawls
• Skip non-HTML payloads by default

Scope

• internal/parse:
  • Streaming parse (net/html and/or goquery on a Reader)
  • Extract absolute links (respect base tags)
  • Sniff Content-Type + size guardrails
• Config flag to allow binary downloads

Acceptance Criteria

• Heap profile shows fewer allocations vs baseline
• Tests cover: base href, meta refresh, unusual encodings

Tasks

• [ ] Implement streamed extraction
• [ ] Add content-type guards
• [ ] Unit tests with fixture pages