Rename Go-style method names in production code (16 offenders)

[sspaink]

Checkstyle's MethodName flags 16 methods in src/main that don't match ^[a-z][a-zA-Z0-9]*$. They split into two groups, and the right fix differs.

Group 1 — public APIs that need a real rename (4)

These are public symbols other callers depend on. Each rename is a breaking change; either land with a deprecation shim if we care about source compatibility, or take the clean break.

• Metrics.Clear() → clear() — already shipped in PR fix: rename Metrics.Clear to clear #70
• NoOpMetrics.Instance() → instance() (opa-evaluator/src/main/java/io/github/open_policy_agent/opa/metrics/NoOpMetrics.java:11)
• QueryTracer.TraceEvent(Event) → traceEvent(Event) (opa-evaluator/src/main/java/io/github/open_policy_agent/opa/tracing/QueryTracer.java:6)
• BufferedQueryTracer.TraceEvent(Event) → traceEvent(Event) (opa-evaluator/src/main/java/io/github/open_policy_agent/opa/tracing/BufferedQueryTracer.java:20)

Group 2 — builtin implementations using OPA's snake_case (12)

These mirror Rego/OPA builtin names; renaming them would diverge from upstream.

• ArithmeticBuiltins: ceil_f, round_f, floor_f, abs_f
• JsonBuiltins: marshal_with_options, is_valid, match_schema, verify_schema
• TokenBuiltins: _verifyECDSA, _verifyEdDSA, _verifyRSASSA, _verifyHMAC (all private)

Decision needed: rename to camelCase, or suppress MethodName on **/ast/builtin/impls/** so the builtin code can stay aligned with OPA's naming?

My read: suppress MethodName for *Builtins.java (matches OPA's surface), rename Group 1 (Java callers, no upstream alignment argument).