Emit structured BuilderProblems from maven-compiler-plugin for build report integration

## Context

PR #12572 (Build Report Foundation) introduced a structured problem reporting system using `BuilderProblem` with `key`, `suggestion`, `documentationUrl`, and severity. Currently, plugin warnings are captured via SLF4J WARN-level log interception, which loses structured metadata.

Plugins should emit structured `BuilderProblem` objects directly to the build report system, starting with `maven-compiler-plugin` as the highest-impact target.

## Problem

Today, compiler warnings flow through:
```
javac DiagnosticCollector → compiler plugin Log.warn(text) → SLF4J → BuildReportCollector
```

This means:
- Compiler warnings appear as flat text with auto-generated keys
- No structured source location from the compiler diagnostic
- No per-warning deduplication control

## API Design — Resolved

The plugin API question was resolved with a new injectable service in Maven 4.1.0:

```java
@Inject DiagnosticReporter diagnosticReporter;
diagnosticReporter.report(BuilderProblem.builder()
.key("compiler:compiler.warn.unchecked:src/main/java/Foo.java:42")
.severity(BuilderProblem.Severity.WARNING)
.message("unchecked cast")
.source("src/main/java/Foo.java")
.lineNumber(42)
.columnNumber(15)
.build());
```

`DiagnosticReporter` is a simple injectable service that pipes `BuilderProblem` objects directly into the `DefaultDiagnosticCollector`.

## Implementation Status

### ✅ Done — maven-compiler-plugin (PR apache/maven-compiler-plugin#1101)

The compiler plugin now maps each `javax.tools.Diagnostic` to a Maven `BuilderProblem` and reports it via `DiagnosticReporter`:

- **Per-location dedup keys**: `compiler:::` — each unique file+line gets its own entry in the build report. 50 unchecked warnings across 50 files produce 50 separate entries (not collapsed into one).
- **Source location**: `source`, `lineNumber`, `columnNumber` from the compiler diagnostic
- **Severity mapping**: `Diagnostic.Kind.ERROR` → `ERROR`, `WARNING`/`MANDATORY_WARNING` → `WARNING`, others → `INFO`
- **Suppression**: Users can suppress via `-Dmaven.diagnostic.suppress=compiler:*` (all) or `compiler:compiler.warn.unchecked` (specific prefix)
- **No behavior change**: Existing logging behavior is unchanged — `DiagnosticReporter` is additive

- #12572 (Build Report Foundation)
- #12643 (Structured BuilderProblem piping in Maven core)

Start by reviewing PR apache/maven-compiler-plugin#1101 and the dependency issues #12572 and #12643, since the compiler-plugin implementation is marked done and this issue lists only future plugin work. Done would require a specifically scoped remaining target, such as surefire, enforcer, dependency, or javadoc integration, with its relevant entry points and tests identified.

Written by the indexing model from the issue text.