Extract SQL-plan instrumentation into standalone repo

[minskya]

Move the operator-timing instrumentation feature out of dataflint/spark into
a self-contained project seed (instrumentation-repo/) that depends on the
published io.dataflint:dataflint-common artifact and publishes to GitHub
Packages.

Removed from spark-plugin:

• common: TimedExec, MetricsUtils, Accelerator, GraphDurationAttribution
• pluginspark3/pluginspark4: DataFlintInstrumentationExtension
• pluginspark3/src/test: the entire instrumentation regression suite
• pluginspark4 build block pulling in the moved DataFlintCodegenFallbackSpec

Kept as the loose-coupling contract in dataflint-common:

• INSTRUMENT_* config constants + registerInstrumentationExtension
  (loads the extension by class name via spark.sql.extensions)
• api.ResolvedStageGroup

The new repo seed compiles (spark3/spark4 + test suite) and assembles fat
jars; the trimmed spark-plugin (common/spark3/spark4) still compiles.
See INSTRUMENTATION-EXTRACTION.md and instrumentation-repo/NEW-REPO-SETUP.md.

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