refactor(lexer): harden safety of transmute

[overlookmotel]

Follow-on after #10933. It's preferable to include explicit type params in calls to mem::transmute rather than relying on type inference. This ensures any later changes won't inadvertently cause UB.

Also expand the safety comment.

[overlookmotel]

• perf(parser): make Kind::Eof (default) 0 #11014
• refactor(lexer): harden safety of transmute #11013 👈 (View in Graphite)
• style(lexer): reformat comments #11012
• main

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #11013 will not alter performance

_{Comparing 05-13-refactor_lexer_harden_safety_of_transmute (c0b68eb) with main (cc0112f)}

Summary

✅ 36 untouched benchmarks

[Boshen]

Merge activity

• May 13, 9:15 PM EDT: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• May 13, 9:15 PM EDT: Boshen added this pull request to the Graphite merge queue.
• May 13, 9:22 PM EDT: Merged by the Graphite merge queue.