Use TRUNCATE_TO_YEAR in the FRBP 9037 example

[jzonthemtn]

The FRBP 9037 example says "dates to year only" but compiled dates to TRUNCATE. TRUNCATE keeps a set number of characters at one end of the value and replaces the rest; reducing a date to its year is TRUNCATE_TO_YEAR.

• 04-frbp-9037.phisql and 04-frbp-9037.json now use TRUNCATE_TO_YEAR.
• The TRUNCATE description in catalog/strategies.yaml said "e.g., dates to year only", which is likely where the example's mistake came from. It now describes what TRUNCATE does and points to TRUNCATE_TO_YEAR. The description is informative only, so what a compiler accepts is unchanged.
• The example's header comment now says the policy is broader than the rule: FRBP 9037(a) limits only birth dates to the year and only minors' names to initials, but PhiSQL cannot yet tell those apart, so the example truncates every date and abbreviates every surname.

No grammar, schema, or compile-contract change, so no RFC is needed (CONTRIBUTING: a bug fix where the implementation diverged from the spec).

Verified locally: scripts/validate_spec.py passes, pytest scripts/tests passes (7), the Python reference compiler's tests pass (326, including test_examples_complete, which compiles each example and compares it to its JSON), and the Java reference build mvn verify passes.

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