feat(report): add v0.6 finding identity

[stacknil]

Summary

• add deterministic finding_id and 1-based episode_index to JSON findings
• bump the report contract from loglens.report.v2 to loglens.report.v3
• lock separated same-subject bursts with a two-finding golden fixture
• document the v0.6 Detection Episode Semantics policy and v2-to-v3 migration

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
• privacy and secret-pattern scan for touched files

Boundaries

• no new detection rules
• no compromise verdict
• no attribution
• no blocking recommendation
• no cross-host correlation