docs: establish LESSONS.md from merged-work retrospective

[e54-bot]

Summary

Establishes docs/LESSONS.md from a merged-work retrospective backfill (window 2026-09-16 → 2026-10-16), following the wrightkit-retro skill in wrightkit/.agents. Each entry names a failure class as a prevention rule, cites the merged-PR evidence, and picks a prevention home.

Entries

• A version-gated feature must define every negotiated version state — Refs feat(protocol): negotiate artifact formats in lpp/compile #40, feat(protocol): add name lookup via lpp/lookup #46.
• A contract check must be exercised on both sides of the boundary — Refs refactor(conformance): re-home misplaced logic across tooling seams #51, feat(conformance): mark provider-supplied leaves in expected responses #53; fast recurrence (two independent PRs, same day).
• A parity refactor must pin the baseline behavior it preserves — Refs refactor(conformance): decompose god files and single-source fixture boilerplate (−32% LOC) #43.
• The reference implementation needs adversarial-input coverage, not only happy-path fixtures — Refs fix: adversarial audit findings (#62–#83) #86 (audit issues Mock provider: unrecognized bytes become identifiers; non-ASCII input panics #62–Docs: correct ADR README references and CHANGELOG/ADR-0006 date ordering #83).

Escalations

• Recurring defect class: contract-enforcement logic merges unproven on both sides of the boundary #89 — recurring defect class for the two-sided-enforcement entries (refactor(conformance): re-home misplaced logic across tooling seams #51 + feat(conformance): mark provider-supplied leaves in expected responses #53 within a rolling three-day window).