Use relative imports for same-directory siblings; split utils into folders

Two related cleanups landed together because they touch the same pattern:

1. Intra-module imports now use relative paths rather than the module's
   own path alias. rng/seeded.ts imports from './derive', not 'rng/derive';
   time/gameTime.ts from './calendar', not 'time/calendar'; the utils
   barrel from './result' etc. This matches rules/01-code-style.md:
   "Relative imports only for same-directory siblings." The alias form
   is reserved for imports *between* modules — how external callers refer
   to a module's public surface.

   (The TS language server in the IDE had trouble resolving the self-
   aliased form even though svelte-check accepted it; this change
   removes the ambiguity.)

2. utils/ is now folder-per-concept. Each utility owns a directory with
   its files broken up into small pieces:

     utils/result/   types, constructors, predicates, map, unwrap
     utils/assert/   assert, assert-never, assert-defined
     utils/equal/    deep
     utils/types/    brand, deep-readonly, element-of, json, non-empty-array
     utils/log/      log

   utils/index.ts re-exports from each subfolder via the barrel. External
   callers import from 'utils' unchanged; internal references are relative.

   One reason to prefer folders over flat files here is headroom — when a
   concept grows, new files sit alongside their siblings inside the
   concept's folder rather than crowding the utils/ root.

No API changes. All 128 tests still green.

1 files changed, 10 insertions, 0 deletions

diff --git a/utils/assert/assert.ts b/utils/assert/assert.ts
new file mode 100644
index 0000000..1fd70c8
--- /dev/null
+++ b/utils/assert/assert.ts
@@ -0,0 +1,10 @@
+/**
+ * Throw if a condition is false, narrow the type to truthy otherwise.
+ * For programmer errors (invariants that should never fail); use
+ * Result<T, E> for recoverable failures.
+ */
+export function assert(condition: unknown, message?: string): asserts condition {
+  if (!condition) {
+    throw new Error(message ?? 'Assertion failed')
+  }
+}