chore: drop experimental feature recursive-nix

This was discussed as part of https://git.lix.systems/lix-project/lix/issues/767
with a one month long comment period.

This commit removes the recursive-nix feature from Lix. It has seen
limited usage and is difficult to stabilize, especially on non-Linux
systems where its behavior is underspecified.

Maintaining this feature complicates core work on the store, as we must
account for the potential presence of the daemon in the sandbox, adding
unnecessary complexity. Additionally, its inclusion in the
platform-independent local store creates risks for non-Linux platforms.

For more details on this removal, refer to the release note entry or the
issue entry.

Change-Id: I9137202f563c0a317f9c5da79cd9fd07d801427a
Signed-off-by: Raito Bezarius <raito@lix.systems>

doc/manual/rl-next/drop-recursive-nix.md

@@ -0,0 +1,23 @@
+---
+synopsis: "Removal of the `recursive-nix` experimental feature"
+issues: [fj#767]
+cls: [2872]
+category: "Breaking Changes"
+credits: ["raito"]
+---
+
+The `recursive-nix` experimental feature and all associated code have been removed.
+
+`recursive-nix` enabled running Nix operations (like evaluations and builds) *inside* a derivation builder. This worked by spawning a temporary Nix daemon socket within the build environment, allowing the derivation to emit outputs that appeared in the outer store. This was primarily used to prototype **dynamic derivations** (dyndrvs), where build plans are generated on-the-fly during a build.
+
+However, this approach introduced critical issues:
+
+- It entrenched the legacy Nix daemon protocol as part of the derivation ABI, which is a blocker for future stabilization.
+- It imposed tight coupling between sandbox setup code and knowledge of Nix internals, complicating refactoring and long-term maintenance.
+- It was never intended to be the final design for dynamic derivations. The original Nix implementation team, who are leading dyndrv development, have agreed it will be replaced (likely via `varlink` or similar) before any stabilization.
+- There is currently no known usage of `recursive-nix` on `lix` or elsewhere **in production**.
+
+If you're using `recursive-nix` for something niche or experimental, we'd love to hear from you on the RFD issue.
+You can still run `nix` inside a builder manually if needed — including with isolated user namespaces and fake stores — but the special daemon-handshake machinery is gone.
+
+This removal unblocks several important internal cleanups.