docs: clean out merge_envs / flatten_visible_bindings references (#6331 follow-up)

[gunbai-bot]

Docs cleanup paired with #6331 (which deletes `flatten_visible_bindings` + `merge_envs` — one-level visibility by construction). Sweeps every doc reference so nothing implies the deleted functions are still live code.

Changes (docs/comments only — no code):

• DESIGN.md §6 + design_document.dag mirror — name-drop; kept the git-history receipt ("a 6-line root fix cut reconcile 81%→6%", which the preamble explicitly sanctions) minus the deleted fn name.
• docs/plans/fold-ergonomics.md + dag/gunbc/plans/fold_ergonomics.dag — name-drop the thesis header.
• docs/plans/post-engine-pr-roadmap.md — the flatten "loaded gun" is noted as removed; the per-module re-materialization root (the SymbolIndex target) stays.
• docs/plans/func-env-sigs-single-authority.md — drop merge_envs from the untouched-list.
• docs/plans/enforcement-intent-design.md — repoint the R1 negative test to the surviving green_merge_envs_base_acc fixture.
• docs/plans/type-env-single-authority-design.md — note flatten is now deleted; the precompute is the remaining materialization this doc targets.
• docs/plans/compiler-algorithm-survey-2026-07-04.md — superseded banner (dated snapshot kept verbatim).

Merge ordering: a few edits reference #6331's deletion, so this should merge with or after #6331. Held as draft until then.

🤖 Generated with Claude Code

[gunbai-bot]

Parked as draft pending #6331. This is the docs cleanup for #6331's flatten_visible_bindings/merge_envs deletion, so it merges with-or-after #6331.

It also can't go green until #6331 lands: it's based on pre-#6331 main, whose floor still times out (the crisis #6331 fixes), and it touches DESIGN.md — a generated-artifact carrier deliberately excluded from the docs-only floor skip (its drift guard is floor-resident). So a full floor runs and times out at 10 min. That's expected, not a defect in the docs (the .dag edits are parse-clean).

Plan: once #6331 merges, rebase onto main (fast floor) → CI green → flip to ready. Nothing to push until then. — sent from lively-raven-355

[gunbai-bot]

Closing this premature draft (the auto-committer opened it before its dependency merged). Not abandoned — sequenced.

This docs cleanup is a follow-up to #6331: it references #6331's flatten_visible_bindings/merge_envs deletion, and it can't pass CI until #6331 merges (pre-#6331 floor times out; DESIGN.md is deliberately excluded from the docs-only skip). Opening its PR before #6331 is the wrong order.

The work is complete and preserved on branch docs-cleanup-post-6331. The moment #6331 merges I'll rebase onto main (fast floor) and re-raise → CI green → merge. — sent from lively-raven-355