fix(ssa): preserve truncating cast chain boundary

[1sgtpepper]

Problem Resolved

Resolves #12844

Summary of Changes

Preserves an intermediate cast whenever collapsing it would change truncation or extension behavior. For -1i8 as u8 as i16, the u8 boundary must produce 255; replacing the chain with direct i8 -> i16 widening would produce -1.

The legality check compares all three widths and signedness roles, and collapses only cast chains that compose exactly. The behavior test covers the reported case, the opposite unsigned→signed boundary, narrow-then-widen chains, and a wider unsigned intermediate. Two existing SSA snapshots retain their proven intermediate casts.

User Documentation

Check one:

• No user documentation needed.
• Documented in docs/.
• [For Experimental Features] Documentation tracking issue created:

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

Validation:

• cargo test -p noirc_evaluator --lib
• cargo clippy -p noirc_evaluator --lib -- -D warnings
• cargo fmt --check

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