[Epic] Conformance verification: phased roadmap (tracking)

[laskoviymishka]

Summary

Tracking (epic) issue for the Apache Iceberg verification framework. It anchors
the work back to the design doc and the community discussion/vote that created
this repository, and lays out how we're phasing the effort.

• Design doc: Shared conformance fixtures for Apache Iceberg implementations —
  https://docs.google.com/document/d/1Qr82hccxJhWxKRT7B-3ylhOLSaJTYfc7hU51-mhzK6Y/edit
• [DISCUSS] thread: https://lists.apache.org/thread/8wsvnhktw27h0tqnxljzmnsogwd52tjj
• [RESULT][VOTE]: https://lists.apache.org/thread/98ntkfmtrqhfj5w54x27tnxfq14fftqb

Approach

We're starting with a minimal, viable core so the repository delivers value
early, and iterating from there. The design doc remains the plan of record; the
phases below are how we sequence toward it, not a change of direction. Scope
expansion (notably writer conformance) is anticipated, not deferred
indefinitely.

Phase 1 — Foundations + read conformance (current)

Goal: establish the repository, the fixture format, the contribution model, and
the first read-conformance surfaces so implementations can begin opting in.

• Repository scaffolding (LICENSE, NOTICE, .asf.yaml, README)
• Repository layout + fixture format conventions
• CONTRIBUTING.md — layout, fixture format, how to add/correct a fixture
• Type conformance fixtures + JSON Schema validation — Add type conformance fixtures and validation #9
• Golden reference tables — Add Reference Tables #6
• Schema fixtures + assertions — Adding Fixtures and Assertions for Schemas #3
• ReadRestrictions fixtures — Add Fixtures for ReadRestrictions #8

Phase 2+ — Expansion (later)

Tracked here for visibility; individual issues to be split out as Phase 1
stabilizes.

• Writer conformance (implementations emit artifacts; a reference reader
  verifies them)
• Additional read surfaces beyond types/schema
• Runner ergonomics and adoption guidance (pinning to a commit, opting into
  surfaces)
• Further surfaces per the design doc

Out of scope (per design doc)

Engine query results, live protocol behavior, and physical encoding choices such
as compression codec and file-format writer version. Any spec-valid encoding is
valid.

Notes

This is a living issue — sub-issues and PRs should link back here. Feedback on
the phasing is welcome.