fix(build): attribute a source to its most specific package root

Package roots can nest — a workspace member lives under the workspace root — and
taking the first matching root filed the member's sources under the outer
package, i.e. into the wrong cache key. Index payloads live in the xpkgs store
and cannot be shadowed this way, so no cached entry is affected today; resolving
by specificity rather than by iteration order is what keeps that true if roots
ever move.

Author: Sunrisepeak

src/build/prepare.cppm

@@ -3885,15 +3885,28 @@ prepare_build(bool print_fingerprint,
         // Sources belonging to each package, package-root-relative and sorted.
         std::vector<std::vector<std::string>> pkgSources(packages.size());
         for (auto& cu : ctx.plan.compileUnits) {
+            // Longest matching root wins. Package roots can nest — a workspace
+            // member lives under the workspace root — and taking the first match
+            // would file the member's sources under the outer package, putting
+            // them in the wrong key. (Index payloads live in the xpkgs store and
+            // cannot be shadowed this way, so no cached entry is affected today;
+            // resolving it by specificity rather than by iteration order is what
+            // keeps that true if roots ever move.)
+            std::size_t best = packages.size();
+            std::size_t bestLen = 0;
+            std::string bestRel;
             for (std::size_t p = 0; p < packages.size(); ++p) {
                 std::error_code ec;
                 auto rel = std::filesystem::relative(cu.source, packages[p].root, ec);
                 if (ec || rel.empty()) continue;
                 auto rels = rel.generic_string();
                 if (rels.starts_with("..")) continue;
-                pkgSources[p].push_back(rels);
-                break;
+                auto len = packages[p].root.generic_string().size();
+                if (best == packages.size() || len > bestLen) {
+                    best = p; bestLen = len; bestRel = std::move(rels);
+                }
             }
+            if (best != packages.size()) pkgSources[best].push_back(std::move(bestRel));
         }
         for (auto& v : pkgSources) std::ranges::sort(v);