Conditionally enable OpenRewrite per module via marker files

[gnodet]

Follow-up to #23041 which disabled OpenRewrite entirely because it quadrupled CI build times.

This re-enables OpenRewrite using Maven's file-activated profiles, scoped to only the modules that have changed Java files. Most builds skip OpenRewrite entirely, and when it does run, it only processes the affected modules.

How it works

1. regen.sh detects which modules have modified .java files (via git diff HEAD~1 HEAD --name-only)
2. Creates a .rewrite-enabled marker file in each affected module
3. The rewrite profile in parent/pom.xml uses <activation><file><exists>.rewrite-enabled</exists></file></activation> — Maven evaluates this per-module, so OpenRewrite only runs where the marker exists
4. No -Prewrite flag needed — everything is automatic

This approach is recipe-agnostic: adding new OpenRewrite recipes to the profile requires no changes to the detection logic.

For shallow clones in CI, --deepen=1 fetches just the parent commit. For PRs, HEAD~1 is the base branch tip (first parent of the merge commit). For main builds, it's the previous squash-merged commit.

Benchmark results (from earlier test PRs)

PR	OpenRewrite	Build time (JDK 21)
#23068 (no FQCNs)	skipped	~22 min
#23069 (with FQCNs)	triggered	~30 min

~8 minutes saved (~36% faster) on builds that don't introduce FQCNs.
With the per-module approach, even builds that trigger OpenRewrite will be faster since it only processes changed modules instead of all ~500.

Test plan

• Tested module detection locally against real commits
• Correctly identifies affected modules (e.g. components/camel-telemetry)
• No modules detected for non-Java changes (dep bumps)
• Handles shallow clones (--deepen=1)
• .rewrite-enabled added to .gitignore
• Verify CI passes with this change

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[github-actions]

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[gnodet]

Claude Code on behalf of Guillaume Nodet

Benchmark results

Tested with two companion PRs to measure the impact:

PR	OpenRewrite	Build time (JDK 21)
#23068 (no FQCNs)	skipped	~22 min
#23069 (with FQCNs)	triggered	~30 min

~8 minutes saved (~36% faster) on builds that don't introduce FQCNs, which is the common case (dependency bumps, doc changes, clean code).

When FQCNs are detected, OpenRewrite runs as before — no regression in correctness.