fix(theme): #66 fail-closed theme-state errors at mkFlake/read

Missing, empty, or non-object theme-state.json now throws a short
message pointing at the template and `nomarchy-theme-sync validate`
instead of a raw readFile/fromJSON stack. Shared reader:
modules/theme-state-read.nix — wired in theme.nix, plymouth/timezone/
hardware, and lib.mkFlake (seq-forced early gate). Field-level schema
in theme.nix kept; unknown border roles still fail there; resolveColor
no longer falls through on missing palette keys. Defaults added for
settings.displayProfile / displayProfileAuto.

Verified: V0 nix flake check --no-build; negative evals (missing,
empty, [], bad border role); good-path template mode + displayProfile
defaults.

agent/MEMORY.md

@@ -71,6 +71,12 @@ iteration would otherwise rediscover.
 - `theme-state.json` is git-tracked inside an 86 MB flake tree, so every
   state write re-copies the source before eval — the wallpapers-artifact
   split (BACKLOG LATER) is the decided fix (§ Faster switches).
+- **Friendly theme-state load** (`modules/theme-state-read.nix`, #66):
+  `builtins.tryEval` does **not** catch `readFile`/`fromJSON` failures —
+  gate with `pathExists` + empty/non-object checks before `fromJSON`.
+  Subtle JSON syntax errors still surface from nlohmann (line/col);
+  field schema stays in `theme.nix`. mkFlake must `builtins.seq` the
+  check onto the whole return set or lazy attr access skips it.
 
 ## Design invariants
 - **Waybar status is never color-only** (item 28 sweep, iteration #69):