reference-docs: build again with 40 workspace crates

The crate README rewrite was unrolled at eval time: one sed block per
crate, each carrying one -e expression per other crate. The installPhase
string grew with the square of the crate count, and at 40 crates
(d5de1e494 added swarm-claude-login) it reached 131708 bytes. Linux
caps a single environment variable at 128 KiB, so exec of the builder
failed with "Argument list too long" and every consumer of
reference-docs failed with it, the website repo included.

The rewrite is now a shell loop over $members at build time, building
the same sed expressions per crate. The phase text no longer depends on
the crate count. Output is byte-identical to the old form at 6aab2ebb6
(39 crates, the last commit where it built).

Closes #5321

nix/packages/reference-docs.nix

@@ -1,6 +1,5 @@
 {
   stdenv,
-  lib,
   self,
   optionsMd,
 }:
@@ -34,22 +33,6 @@
 let
   cargoToml = builtins.fromTOML (builtins.readFile ../../Cargo.toml);
   members = cargoToml.workspace.members;
-
-  # A member-name relative link (`../<other-crate>`, no specific file —
-  # forge's browser renders that as a directory listing today) becomes a
-  # sibling crates/<other-crate>.md link once every crate README
-  # lives flat in the same directory. Built once, applied per file below,
-  # rather than a fresh `lib.concatMapStrings` per crate's own transform —
-  # every crate needs the exact same substitution list (every *other*
-  # member), so this only differs on the `member != other` filter.
-  siblingLinkFixups =
-    member:
-    lib.concatMapStrings (
-      other:
-      lib.optionalString (other != member) ''
-        -e 's,\]\(\.\./${other}(/)?\),](./${other}.md),g' \
-      ''
-    ) members;
 in
 stdenv.mkDerivation {
   pname = "hyperhive-docs";
@@ -63,11 +46,15 @@ stdenv.mkDerivation {
   # the alternative is hand-copying READMEs into docs/ and letting them
   # drift, which is the exact duplication this issue exists to avoid.
   src = ../../docs;
-  inherit self optionsMd;
+  inherit self optionsMd members;
 
   dontBuild = true;
   dontConfigure = true;
 
+  # The per-crate rewrite is a loop at build time, so this phase's text
+  # stays the same size as crates are added: each attribute reaches the
+  # builder as one environment variable, and Linux refuses to exec with
+  # a single one over 128 KiB.
   installPhase = ''
     runHook preInstall
     mkdir -p $out
@@ -75,24 +62,24 @@ stdenv.mkDerivation {
     mkdir -p "$out/crates"
     cp -r "$optionsMd" "$out/options"
 
-    ${lib.concatMapStrings (member: ''
-      if [ -f "$self/${member}/README.md" ]; then
-        # Rewrite relative links that were correct from the crate's own
-        # position in the repo tree but aren't once the file is
-        # virtually one level under docs/ instead:
-        #   ../docs/x.md      (crate root -> repo docs/)   -> ../x.md  lint:allow (stand-in name)
-        #     (crates/ -> docs/ is one directory shallower than
-        #     <crate>/ -> docs/ was, so the leading docs/ segment drops)
-        #   ../<sibling-crate> (that crate's own directory, no file)
-        #     -> ./<sibling-crate>.md, once that sibling also has a
-        #     crates page every crate README lives flat beside it.
-        sed -E \
-          -e 's,\]\(\.\./docs/,](../,g' \
-          ${siblingLinkFixups member} \
-          "$self/${member}/README.md" \
-          > "$out/crates/${member}.md"
-      fi
-    '') members}
+    for member in $members; do
+      [ -f "$self/$member/README.md" ] || continue
+      # Rewrite relative links that were correct from the crate's own
+      # position in the repo tree but aren't once the file is
+      # virtually one level under docs/ instead:
+      #   ../docs/x.md      (crate root -> repo docs/)   -> ../x.md  lint:allow (stand-in name)
+      #     (crates/ -> docs/ is one directory shallower than
+      #     <crate>/ -> docs/ was, so the leading docs/ segment drops)
+      #   ../<sibling-crate> (that crate's own directory, no file; forge
+      #     renders it as a directory listing) -> ./<sibling-crate>.md,
+      #     since every crate README lives flat beside it under crates/.
+      fixups=(-e 's,\]\(\.\./docs/,](../,g')
+      for other in $members; do
+        [ "$other" != "$member" ] || continue
+        fixups+=(-e "s,\]\(\.\./$other(/)?\),](./$other.md),g")
+      done
+      sed -E "''${fixups[@]}" "$self/$member/README.md" >"$out/crates/$member.md"
+    done
 
     runHook postInstall
   '';