feat: add --incremental flag to skip up-to-date page object generation

[intermezzio]

Closes #244

Summary

• Adds an opt-in --incremental (-I) CLI flag (and matching JsonCompilerConfig constructor parameter) that causes the compiler to skip generation for page objects whose generated .java artifacts are newer than the source .utam.json.
• New TranslatorTargetConfig.isUpToDate(...) interface method with a default return false, preserving behavior for downstream custom target configs.
• DefaultTargetConfiguration overrides isUpToDate with a per-page-object timestamp check (interface and impl .java paths must exist and be ≥ source mtime; impl-only POs only require the impl artifact).
• DefaultTranslatorRunner.write() consults isUpToDate before invoking getGeneratedCode() (which is where google-java-format runs) and skips the interface, class, JSON resource, and unit-test writes for that PO. Aggregate outputs (manifest, dependency-injection config) still cover every PO.

Motivation

When this library is consumed via the compiler plugin in a downstream project, mvn triggers an all-or-nothing rebuild on every invocation, with google-java-format dominating the cost. Incremental mode skips the formatter and file writes for page objects whose targets are already current, which is a large speedup on iterative dev loops.

Behavior

• Default off; existing CLI/JSON-config callers unchanged.
• With --incremental: a PO is skipped iff its generated targets exist and have mtime ≥ the source .utam.json mtime. Any missing or older target forces regeneration. Source mtime ≤ 0 also forces regeneration (matters for unit tests with synthetic sources).

Test plan

• New tests in DefaultTargetConfigurationTests covering isUpToDate for: flag off, source mtime zero, missing interface file, missing class file, source newer than targets, targets newer than source, interface-only PO, impl-only PO.
• New tests in DefaultTranslatorRunnerTests verifying write() skips all POs when target reports up-to-date and writes only the out-of-date PO when the target is selective.
• New test in JsonCompilerConfigTests verifying the 4-arg constructor wires the incremental flag through to the resulting TranslatorTargetConfig.
• mvn -pl utam-compiler test — 644/644 pass.
• mvn -pl utam-core test — 385/385 pass.
• mvn spotless:check clean.

[salesforce-cla]

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[salesforce-cla]

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