feat(build-time-versioning): nix packages compute build time at build phase

First attempt used builtins.currentTime at flake-evaluation time -- this
broke the plain, documented `nix build .#xcecore_debug` command entirely
(pure-eval strips builtins.currentTime, requiring --impure) and produced
a raw epoch-seconds string instead of UTC ISO-8601. Caught by task-local
review (REJECTED, round 1) before merge.

Fixed by moving the computation into each derivation's own build phase:
a preConfigure hook in xce.nix/xcecli.nix runs `date -u` (stdenv's
bundled coreutils, unaffected by pure-eval) and appends -Dbuild_time= to
mesonFlagsArray, with a fail-loud guard against empty output. commit_id's
existing self.shortRev/self.dirtyShortRev eval-time mechanism is
untouched. Verified via nix eval (no --impure) and nix build reaching
configure with real values on both xcecore_debug and xcecli (round 2,
APPROVED).

Task 2.2/13 of build-time-versioning. Also corrects design.md/research.md,
which described the now-superseded builtins.currentTime approach.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/nixSupport/xcecli.nix

@@ -25,6 +25,17 @@ stdenv.mkDerivation {
 
   impureEnvVars = [ "HOME" ];
 
+  # See xce.nix for why build_time is computed here (build phase) rather
+  # than at flake-evaluation time.
+  preConfigure = ''
+    buildTime="$(date -u +%Y-%m-%dT%H:%M:%SZ)"
+    if [ -z "$buildTime" ]; then
+      echo "xcecli.nix: date -u produced an empty build time -- the -Dbuild_time= meson option would not reach the binary" >&2
+      exit 1
+    fi
+    mesonFlagsArray+=("-Dbuild_time=$buildTime")
+  '';
+
   mesonFlags = [
     "-Dcomponent=xcecli"
     ("-Dcommit_id=" + myCommitId)