docs(rfc-0002): dataset immutability principle + reject --append (cli#156)

[LukasWodka]

What

Records the dataset-immutability principle in RFC-0002 and turns the --append non-goal from "tracked separately" into an explicit rejection, now that cli#156 is closed obsolete.

Two edits to docs/rfcs/0002-data-ingest-flow.md:

• §2 Principles — new Principle 6: datasets are immutable snapshots. A dataset is the fixed basis for its use cases + training; reproducibility depends on it not changing under already-trained models. Ingest creates or wholesale-replaces (--overwrite); never appends in place. Growth = a new (versioned) dataset. (The RFC never actually stated this principle before, even though the whole flow depends on it.)
• §13 Non-goals — --append rewritten from "tracked separately, cli#156" to rejected, with the integrity/reproducibility rationale and the pointer to backend#1073 (the canonical decision record) + cli#156 (closed obsolete 2026-07-11).

Why

cli#156 (data ingest --append) was closed as obsolete — in-place append breaks the reproducibility guarantee the leaderboard depends on. The RFC-of-record still framed append as a pending/tracked item, which was misleading. This makes the design-of-record match the decision.

Docs-only (the RFC markdown); no code change. Rev 5.

---

Note

Low Risk
Markdown RFC updates only; no runtime or API behavior changes.

Overview
RFC-0002 now states that datasets are immutable snapshots as Principle 6: ingest only creates or wholesale-replaces (--overwrite); growth is a new versioned dataset, not in-place mutation.

§13 no longer treats server-side data ingest --append as tracked work—it is explicitly rejected, aligned with cli#156 (closed obsolete) and backend#1073. The rationale is reproducibility and leaderboard integrity after training.

Docs-only (Rev 5); no code changes.

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