dsc#103: pin bare-unsafe Err typing with a checker-level test

[samifouad]

What

Follow-up to #108 (merged), which carried the bulk of dsc#103. Two pieces landed after that branch was cut and were left uncommitted in the working tree:

• A deka_syntax typeck unit test (bare_unsafe_err_side_is_string) asserting that Err(e) => e.message on a bare unsafe { } is a check-time error — ``string`` has no field `message`` — pinning Result<Infer, string> at the checker level, next to the testsuite fail fixture that pins it end-to-end.
• The unsafe_err_payloads_cross_as_strings doc comment updated to describe the Result<Infer, string> typing instead of the old Result<Infer, Infer>.

No behavior change.

Verified

cargo test -p deka_syntax bare_unsafe_err_side_is_string   # ok
cargo test -p cli --test emit unsafe_err_payloads_cross_as_strings  # ok

-kimi