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which described the now-superseded builtins.currentTime approach.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>

.kiro/specs/build-time-versioning/tasks.md

@@ -17,10 +17,10 @@
   - _Requirements: 1.1, 1.2, 1.3, 1.4, 4.1, 4.2, 5.1, 5.2_
   - _Boundary: xce-build.sh_
 
-- [ ] 2.2 (P) Nix packages compute and pass build time
-  - Add a `builtins.currentTime`-derived build-time value in `src/flake.nix`, passed via a new `-Dbuild_time=` entry alongside the existing `-Dcommit_id=` wiring in `xce.nix`/`xcecli.nix`
-  - Since `builtins.currentTime` cannot itself be empty, the confirmation this requirement asks for is structural here: assert (in the derivation or a post-build check) that the meson option actually reached the binary, rather than a runtime empty-value check
-  - Observable: `nix build .#xcecore_debug` produces a binary whose `--version` shows a non-empty build time, without any `git` invocation inside the sandboxed derivation
+- [x] 2.2 (P) Nix packages compute and pass build time
+  - ~~Add a `builtins.currentTime`-derived build-time value in `src/flake.nix`~~ — corrected during review (round 1 REJECTED): `builtins.currentTime` breaks pure-eval `nix build` (requires `--impure`) and yields non-UTC epoch seconds. Implemented instead as a `preConfigure` build-phase `date -u +%Y-%m-%dT%H:%M:%SZ` hook in `xce.nix`/`xcecli.nix` appending to `mesonFlagsArray`, alongside the unchanged eval-time `-Dcommit_id=` wiring
+  - Fail loudly (non-empty guard) if `date -u` ever produces empty output, before the value reaches `mesonFlagsArray`
+  - Observable: `nix build .#xcecore_debug` produces a binary whose `--version` shows a non-empty, ISO-8601 UTC build time, without any `git` invocation inside the sandboxed derivation, and without requiring `--impure`
   - _Requirements: 1.1, 1.2, 1.4, 4.1, 4.2, 5.1, 5.2_
   - _Boundary: Nix packaging_
 
@@ -122,3 +122,6 @@
   - Observable: the installed wheel's `--version` output differs from an editable (`pip install -e .`) install's `unknown`-only output, proving the generated module actually replaces the fallback
   - _Requirements: 1.1, 1.2, 2.1, 4.1, 4.2_
   - _Depends: 5.1, 5.2_
+
+## Implementation Notes
+- **Task 2.2**: `builtins.currentTime` cannot be used for anything feeding a `nix build` derivation without `--impure` — Nix's flake pure-eval mode strips it entirely, breaking the plain, documented `nix build .#xcecore`/`.#xcecore_debug` commands. It also yields a raw epoch-seconds integer, not UTC ISO-8601. Any nix-side build metadata that must vary per-build (timestamps, etc.) belongs in a derivation's `preConfigure` (or similar) build-phase shell hook using ordinary coreutils (`date -u`), appended to `mesonFlagsArray` — never in eval-time Nix expressions. `self.shortRev`/`self.dirtyShortRev` remain the correct, eval-time-safe mechanism specifically for the commit id (Nix computes those specially even under pure-eval), so this distinction is per-value, not a blanket "avoid nix expressions" rule.