docs(defork-audit): record #6341 execution reality — Root B narrowed to shadow-removal; 3 walls re-map follow-ons

[briansrls]

Follow-up to #6341 (merged). Records the algebra unfork's execution reality in the roadmap-linked de-fork audit §3b, so the upcoming roadmap work picks up the correct follow-on lanes instead of the stale plan.

The audit's §3b still described the original Root B plan (move the ops, unify the definition, promote QualifiedName) and named jolly-cat as Root-A owner. #6341 found — by-execution — that most of that is walled:

• A — reserved builtin-method names block the op-move to std.* (the 8 ops are free-fn duplicates of dag builtins → dissolve into .filter()/.length(), a separate lane).
• B — Root A is UNOWNED (jolly-cat left the tree) and confirmed load-bearing: 05_emit_rust.dag hardcodes 3 op paths for the String=FreeMonoid<Char> host bridge, blocking the String/List alias grounding until Root A is re-dispatched.
• C — QualifiedName is node-gated, not algebra-gated (its body imports v2.std.node/diagnostic/lexing), so its promotion needs the node defork too.

What #6341 did land: the FreeMonoid variant-drop shadow is removed (single dag/std/algebra authority). The §3b addition summarizes the three walls and links algebra-grounding-unification-design.md §9 for the full trace/census — no dual representation, detail stays in §9.

Docs-only; completes the tracking chain roadmap 5-defork → audit §3b → §9.