feat(dataframe): expose the executed physical plan with per-operator metrics

[LantaoJin]

Which issue does this PR close?

• Closes feat(dataframe): expose the executed physical plan with per-operator metrics #96 .

Rationale for this change

df.explain(true, true) already runs the plan and attaches per-operator metrics, but the result is a DataFrame of text rows. Programmatic consumers — query-shape regression tests, operational audit feeds, build-time benchmarks — have to scrape output_rows=12345, elapsed_compute=4.2ms strings out of those rows. Brittle to upstream wording, ergonomically painful, and the typed metric values (Count, Time, Gauge) lose their type along the way.

This PR adds a typed accessor df.executedPlan() that returns an immutable ExecutedPlan tree once the DataFrame has been executed via collect() / executeStream(). Each node carries the operator name, a one-line display rendering, child nodes, and an OperatorMetrics POJO with OptionalLong fields for the well-known metric variants plus a Map<String, Long> for custom counters.

try (DataFrame df = ctx.sql("SELECT count(*) FROM events");
     ArrowReader r = df.collect(allocator)) {
    while (r.loadNextBatch()) { /* drain */ }
}
ExecutedPlan plan = df.executedPlan();
long rows = plan.metrics().outputRows().orElse(-1L);

The contract is post-mortem: executedPlan() requires a prior collect / executeStream and rejects with IllegalStateException("call collect() or executeStream() first") if called pre-execution. A future PR can extend the surface to make pre-execution structure inspection available too — that follow-up is intentionally out of scope here to keep this PR focused on the metric-snapshot surface.

What changes are included in this PR?

• New public records ExecutedPlan and OperatorMetrics.
• New DataFrame.executedPlan() method.
• New proto/executed_plan.proto (ExecutedPlanNodeProto).
• Native side: executed_plan.rs.
• Java-side: one new final long planId field assigned at construction.

Out of scope (deferred to follow-up PRs):

• Per-partition metric breakdown.
• Time/Gauge-shaped custom metrics; v1 surfaces Count-shaped customs only.

Are these changes tested?

Yes. 10 new tests in the ExecutedPlanTest.

Are there any user-facing changes?

Yes, additive only -- no behavior changes for existing callers.

• New public types ExecutedPlan and OperatorMetrics (records).
• New DataFrame.executedPlan() method.