frontend: tighten extraFiles target type to strMatching regex

damocles suggested using lib.types.strMatching for the target option
itself rather than relying solely on the post-hoc assertion. Pattern:
`^[A-Za-z0-9_][A-Za-z0-9_./-]*$` — first char alphanumeric/_, then
alphanumerics + _ + . + / + - allowed (so nested layouts like
"games/bitburner" still work).

This rejects at type-check time:
- leading `/` (absolute paths)
- leading `.` (so `..` as a full string blocked, also `./foo`)
- leading `-` (would parse as flag by some tools)
- spaces, control chars, weird unicode

The existing assertion stays — it catches mid-path `..` segments
(`foo/../bar`) that the regex can't reject without lookahead. POSIX
regex (which nix uses) doesn't support lookahead, so the
type-and-assertion split is the cleanest expression.

Refs #273.

nix/templates/harness-base.nix

@@ -192,7 +192,15 @@
               '';
             };
             target = lib.mkOption {
-              type = lib.types.str;
+              # First char must be alphanumeric/underscore (rules out
+              # leading `/`, leading `.`, leading `-`); inner chars
+              # include `.` and `/` so nested layouts like
+              # `"games/bitburner"` work. This is the shape check —
+              # the `..`-segment traversal check is the assertion in
+              # `config.assertions` below (regex alone can't reject
+              # mid-path `..` segments without lookahead, which nix
+              # POSIX regex doesn't support).
+              type = lib.types.strMatching "^[A-Za-z0-9_][A-Za-z0-9_./-]*$";
               default = name;
               defaultText = lib.literalMD "the attribute name";
               description = ''
@@ -201,6 +209,11 @@
                 the on-disk layout in the merged derivation. Defaults
                 to the attribute name. Use forward slashes for
                 nested layouts (e.g. `"games/bitburner"`).
+
+                Constrained shape: must start with an alphanumeric or
+                `_`, and only contain alphanumerics, `_`, `.`, `/`,
+                `-`. `..` segments are separately rejected at config
+                eval time.
               '';
             };
           };
@@ -452,21 +465,20 @@
         message = "hyperhive.icon must point to an .svg file";
       }
       # hyperhive.frontend.extraFiles[*].target is concatenated into
-      # $out during the mergedDist build. Reject path traversal
-      # (`..` segments) and absolute paths (leading `/`) — both would
-      # let an entry escape the static dir. agent.nix is operator-
-      # reviewed, so this is belt-and-braces, but it's the kind of
-      # mistake that's easy to make and hard to spot during review.
+      # $out during the mergedDist build. The option's strMatching
+      # type already rejects leading `/`, leading `.`, and the
+      # weirder characters; this assertion catches mid-path `..`
+      # segments (e.g. `foo/../etc/passwd`) that the type's regex
+      # can't easily express without lookahead. agent.nix is
+      # operator-reviewed, so this is belt-and-braces — but it's the
+      # kind of mistake that's easy to make and hard to spot.
       {
         assertion = lib.all (
-          entry:
-          !(lib.hasPrefix "/" entry.target)
-          && !(builtins.any (seg: seg == "..") (lib.splitString "/" entry.target))
+          entry: !(builtins.any (seg: seg == "..") (lib.splitString "/" entry.target))
         ) (lib.attrValues config.hyperhive.frontend.extraFiles);
         message = ''
-          hyperhive.frontend.extraFiles: `target` must be a relative
-          path inside the static dir — no leading `/`, no `..`
-          segments.
+          hyperhive.frontend.extraFiles: `target` must not contain
+          `..` path segments.
         '';
       }
     ];