docs: point the fork procedures at the current pin, drop the skew note

[annejan]

Follow-up to #38, from a review of it. Comments and docs only — no code, no dependency change.

The fork skew is gone. martin-tightcut was fast-forwarded to ffaecd70 (the 8.0.1-based rev #38 pins), so the NOTE (2026-08-30) block in Cargo.toml describes a state that no longer exists. The pre-8.0.1 base is kept on the fork as martin-tightcut-8.0.0-base, so nothing was destroyed, and martin-tightcut-rebased still exists too.

What replaces the note is the failure mode that is durable: a ../bgs-fork checkout older than the pinned rev no longer satisfies version = "8.0.1", and cargo does not error on that — it prints warning: patch ... was not used in the crate graph and resolves the real crate from crates.io instead, after which the build dies on missing CloudSettings fields (bulge, swarm, deform_mode, morph_stagger, additive) with the cause scrolled off-screen. Reproduced with real cargo.

OPTIMIZATION-BACKLOG.md named a stale SHA as a gate. The fork-shader procedure said to confirm the local checkout matches "the pinned commit (Cargo.lock rev f94855a4)" before editing and pushing martin-tightcut. That was the pre-8.0.1 rev — the only literal SHA left in the tree, and an instruction rather than a description. Following it after #38 would have confirmed the old base as correct and pushed shader edits from it, reverting the rebase. It now points at whatever Cargo.lock names, so it cannot go stale on the next bump.

DESIGN.md overstated what 8.0.1 contains. It said the §11 lazy_type_alias → checked_type_aliases rename "shipped in 8.0.1". Upstream order is #236 (the version bump) and then #237 (the rename) on top of it, and 8.0.1 was published to crates.io on 2026-06-25, before #237 landed. The registry 8.0.1 still gates on lazy_type_alias and fails E0557 on the pinned nightly, so the fork patch stays load-bearing. #38's own CHANGELOG entry already had this right.

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