Remove the synthetic compatibility JFR output - #27

Summary

The correlator wrote jonoffcpu-offcpu-synthetic.jfr beside the collapsed stacks and the stack profile: duration-quantized jdk.ExecutionSample events for JFR viewers. It carried nothing the other outputs don't. It also needed:

• a patched copy of the JMC flightrecorder writer
• a patch step that no build cache could reuse
• async-profiler's make-built jfrconv to test it, which records the build date and so reruns the correlator's integration tests on every fresh CI checkout

This PR removes the feature and everything that existed only for it.

Correlator

• Deleted: CompatibilityJfrWriter, SyntheticJfrSource and QuantumPlanner, plus Thinning.scaleQuantum and AnalysisOutput.synthetic().
• Sample event time: the samples' event-time column (SampleColumns.startEpochNanos) was read only by the writer, so it's removed. A sample slot now retains 30 bytes instead of 38, and RetentionEstimate is updated to match. A sample with an out-of-range start time is no longer rejected, since the value isn't used.
• Collapsed output: always written for a complete analysis.

CLI

• --quantum-ns and --max-synthetic-events are removed.
• --format takes only collapsed for a complete analysis, and that stays the default. With --partial true it takes diagnostics or collapsed as before. both and jfr are now usage errors (exit 64).

Report schema (jonoffcpu-report.proto)

• The syntheticJfr block (Report field 24) and the coarsenSyntheticQuantum and omitSyntheticJfr degradation steps (5, 6) are removed. Their numbers and names are reserved.
• The degradation ladder now starts by dropping the audit outputs.

Build

• Removed: the JMC writer dependency with its patch and retained licenses, and the extractJmcWriterSources, applyJmcWriterPatch and compileJmcWriterPatch tasks.
• Removed: the org.openjdk.jmc relocation, and the buildAsyncProfilerConverter task with the jfrconv input of integrationTest.
• Unchanged: the published jfr-converter.jar.

Proof tools and docs

• agent-smoke-musl-container.sh and run-agent-signal-pressure.py no longer convert the synthetic JFR.
• Updated to match: README, OFFLINE.md, AGENTS.md, jonoffcpu-native/README.md, the correlator help snapshots, and the architecture diagram, which is rendered again.

Compatibility

• Callers: external callers passing --quantum-ns, --max-synthetic-events or --format both|jfr now get a usage error.
• Report readers: consumers reading syntheticJfr from the report no longer find it.

Verification

• ./gradlew spotlessCheck :jonoffcpu-agent:check :jonoffcpu-correlator:check :jonoffcpu-jfr-converter:check passes on an arm64 Mac: 302 tests, 0 failures, 11 skipped. That includes the musl aarch64 container integration tests.
• The second run reuses the configuration cache.