fix decode_unchecked bug

[pkhry]

Description

[T;N] used incorrect offset for decoding static types, added a regression test

[github-actions]

Benchmark Size Comparison (vs main)

Artifact	Baseline	Current	Delta	Status
fibonacci_builder-dsl.debug.polkavm	37321	37321	+0.00%	OK
fibonacci_builder-dsl.release.polkavm	638	638	+0.00%	OK
fibonacci_no-alloc.debug.polkavm	35940	35940	+0.00%	OK
fibonacci_no-alloc.release.polkavm	595	595	+0.00%	OK
fibonacci_with-alloc.debug.polkavm	46504	46504	+0.00%	OK
fibonacci_with-alloc.release.polkavm	600	600	+0.00%	OK
multi_builder-dsl.debug.polkavm	58597	58605	+0.01%	OK
multi_builder-dsl.release.polkavm	3024	3024	+0.00%	OK
multi_no-alloc.debug.polkavm	60404	60404	+0.00%	OK
multi_no-alloc.release.polkavm	2919	2919	+0.00%	OK
multi_with-alloc.debug.polkavm	72170	72170	+0.00%	OK
multi_with-alloc.release.polkavm	3076	3076	+0.00%	OK
mytoken_builder-dsl.debug.polkavm	56251	56251	+0.00%	OK
mytoken_builder-dsl.release.polkavm	3683	3683	+0.00%	OK
mytoken_no-alloc.debug.polkavm	59429	55535	-6.55%	OK
mytoken_no-alloc.release.polkavm	2923	2923	+0.00%	OK
mytoken_storage.debug.polkavm	65050	61306	-5.76%	OK
mytoken_storage.release.polkavm	3127	3127	+0.00%	OK
mytoken_with-alloc.debug.polkavm	73646	69761	-5.28%	OK
mytoken_with-alloc.release.polkavm	3940	3940	+0.00%	OK
----------------------------------------------------	------------	------------	----------	--------
Total	589837	578322	-1.95%

OK: All artifacts within 5% regression threshold

_{Run 26837781708 | 0a18c3c | 2026-06-02 18:02 UTC}