fix: proptest lambda failures — 'ask' keyword not in valid_identifier blocklist

[noahgift]

Five-Whys Root Cause

1. Why do 5 property tests fail? prop_arrow_syntax_parses, prop_nested_lambdas_parse, etc. panic with assertion failed: result.is_ok()
2. Why does parsing fail? The generated identifier "ask" is now a keyword, so the parser rejects it
3. Why does proptest generate "ask"? The valid_identifier() strategy's keyword blocklist doesn't include "ask"
4. Why isn't "ask" blocked? Commit e650389 added "ask" as a keyword but didn't update the proptest strategy
5. Why wasn't this caught? The previous CI run was green — the proptest seed only hit "ask" on the next run

Also: RUSTSEC-2026-0002 (lru 0.12.5)

Transitive dep via ratatui 0.29.0 → jugar-probar. lru 0.16.3 is available on crates.io.

Fix

• Add "ask" to keyword blocklist in valid_identifier() proptest strategy
• Update ratatui/lru to resolve RUSTSEC-2026-0002

Discovered by kaizen sweep 2026-04-06.