test(state): #133 — pin the #107 legacy-name shim (and its fail-closed gate)

mkFlake prefers state.json and still accepts the legacy theme-state.json,
because a machine only migrates on its next menu write. Nothing evaluated a
checkout carrying ONLY the legacy name, so the shim could rot with every
check green — breaking rebuilds for precisely the users who have not opened
the menu since #107, i.e. the ones least likely to see it coming.

checks.state-legacy-name evaluates two fixtures: legacy-only must resolve,
and no-state-at-all must still throw. The gate is half the shim's contract
(it turns "no state file" into one readable error instead of a readFile
stack from inside the module system) and was equally unpinned.

Cheap by construction: mkFlake wraps its return in `builtins.seq
_themeState`, so forcing the attrset forces the state read and nothing else
— no module system, no home.nix. Hence a one-file fixture and no measurable
cost (flake check still ~35s). That file is a SYMLINK to the shipped
template's state.json: a copy would drift and the check would quietly start
testing a fossil. The fixture also asserts it has no state.json, or it would
pass while testing nothing.

Both assertions proven by breaking lib.nix — deleting the fallback, then
defeating the gate — and watching each fail by name. Delete this check
together with the shim and the nomarchy-theme-sync alias.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

checks/fixtures/legacy-state/theme-state.json

@@ -0,0 +1 @@
+../../../templates/downstream/state.json

checks/fixtures/no-state/README.md

@@ -0,0 +1,5 @@
+Deliberately empty: a checkout with **neither** `state.json` nor the legacy
+`theme-state.json`. `checks.state-legacy-name` asserts `mkFlake` fails closed
+here — the fail-closed gate in `lib.nix` is what turns "you have no state file"
+into one readable error instead of a raw `readFile` stack from deep inside the
+module system.