fix(gettext): coalesce duplicate file changes in incremental msg* tasks

[vlsi]

Summary

• Downstream consumers (e.g. pgjdbc's generateGettextSources) sometimes lost a locale's output: the per-locale messages_<lang>.java was missing from build/gettext/generate_java_resources/po/..., even though <lang>.po was present and unmodified. --rerun-tasks papered it over; the next incremental run reintroduced the loss.

• Root cause: when poFiles aggregates overlapping providers (e.g. files(sourceSets.main.get().allSource).filter { …".po" } plus feature variants like registerFeature("sspi")/("osgi") reusing sourceSets["main"]), Gradle can report the same physical .po file as both ADDED and REMOVED in one invocation. The task body in MsgAttribTask.run (and MsgMergeTask/MsgFmtTask) iterates events in order, so msgattrib writes outDir/ru.po and the following REMOVED branch immediately deletes it. The empty outDir entry cascades downstream and a locale silently disappears.

  Trace (--info):

  Task ':…:remove_obsolete_translations' is not up-to-date because:
    Input property 'poFiles' file …/ru.po has been added.
    Input property 'poFiles' file …/ru.po has been removed.
  

Fix

Introduce coalesceFileChanges(changes, currentSourceFiles) in FileChangeCoalescing.kt and route the three incremental tasks (MsgAttribTask, MsgMergeTask, MsgFmtTask) through it. Per file.absolutePath:

• non-FILE entries are dropped,
• ADDED/MODIFIED wins over REMOVED (collapses the spurious pair to a single ADDED/MODIFIED),
• a pure-REMOVED event is suppressed when the file is still present in the resolved poFiles.files snapshot.

Insertion order is preserved, so the processing order matches the previous behavior.

Tests

Added FileChangeCoalescingTest (8 unit cases — all green):

• ADDED → REMOVED collapses to ADDED
• REMOVED → ADDED collapses to ADDED
• MODIFIED wins over REMOVED
• pure REMOVED suppressed when the file is still in the snapshot
• pure REMOVED kept when the file is absent from the snapshot
• non-FILE (DIRECTORY) entries dropped
• distinct files reported independently
• insertion order preserved across distinct files

I did not add an end-to-end integration test that wires duplicate providers through a real GradleRunner: msgattrib/msgmerge/msgfmt would have to be installed on the test host, and the plugin had no test sources before this PR. The reported regression is purely the duplicate-event handling, which the unit tests cover exactly.

Test plan

• ./gradlew :plugins:gettext-plugin:test --rerun-tasks — 8/8 green
• ./gradlew :plugins:gettext-plugin:build — green
• Run pgjdbc repro from the bug report (./gradlew :postgresql:generateGettextSources --rerun-tasks, delete one messages_<lang>.java, re-run with --info) and confirm the locale output is regenerated and no spurious ADDED + REMOVED pair drops it.

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