builtins: add uniqby keyed dedup (tree-only HOF)

[danieljohnmorris]

uniq did raw equality dedup. uniqby dedupes by a derived key, keeping the first occurrence per key. Critical for record-shaped lists where the equality you care about is on one field.

Implementation:

• tree-only HOF, verifier enforces function literal in fn position
• interpreter evaluates fn per element, tracks seen keys in a hash set
• preserves input order

Tests: cross-engine regression tests + examples/uniqby.ilo with -- run: directives.

[codecov]

❌ 2 Tests Failed:

Tests completed	Failed	Passed	Skipped
3469	2	3467	0

View the top 2 failed test(s) by shortest run time

ilo::regression_uniqby::uniqby_basic_dedup_by_key_tree

Stack Traces | 0.009s run time

thread 'uniqby_basic_dedup_by_key_tree' (18993) panicked at tests/regression_uniqby.rs:66:5:
assertion `left == right` failed
  left: "[\"apple\"]"
 right: "[apple, banana, cherry]"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

ilo::regression_uniqby::uniqby_all_same_key_keeps_first_tree

Stack Traces | 0.012s run time

thread 'uniqby_all_same_key_keeps_first_tree' (18995) panicked at tests/regression_uniqby.rs:88:5:
assertion `left == right` failed
  left: "[\"alpha\"]"
 right: "[alpha]"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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