fix(vm): wire B64Dec into tree_bridge_returns_result

[danieljohnmorris]

Summary

One-line fix: #560 added the standard base64 cluster but missed wiring B64Dec into the returns_result list in src/vm/mod.rs. b64-dec! (with bang) tripped the debug_assert!(tree_bridge_returns_result(builtin)) at line 1887 because the bridge thought it returned a plain value.

Why this matters

examples/crypto-primitives.@ has been failing on main since #560 merged. Every PR's coverage job has inherited the failure. This one-line edit unblocks the whole queue.

Verification

• Build + clippy clean.
• cargo test --release --features cranelift --test examples_engines passes (was panicking).

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
7544	1	7543	0

View the top 1 failed test(s) by shortest run time

ilo::skill_md::body_is_thin_bootstrap

Stack Traces | 0.004s run time

thread 'body_is_thin_bootstrap' (64405) panicked at tests/skill_md.rs:254:5:
SKILL.md body is 10925 bytes; bootstrap shape should stay well under 8 KB
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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