feat(detector): emit time-separated detection episodes

[stacknil]

Summary

• add detector episode semantics so the same rule subject can emit multiple time-separated findings
• keep the existing best-window selection inside each episode candidate
• document that rule_id + subject is no longer unique within a report and that consumers should use window fields plus evidence IDs

Notes

• No new detection rules are added.
• The JSON report remains loglens.report.v2 / schema_version: 2.
• Existing golden report contracts remain stable; new detector tests cover repeated episodes directly.

Validation

• cmake -S . -B build
• cmake --build build
• ctest --test-dir build -C Debug --output-on-failure
• git diff --check
• local Markdown link check for touched docs
• added-line privacy / secret scan