Apply path/subject naming convention to datasourcex + commit ADR 0002

[rossdanderson]

Parent

#65

What to build

Apply the path/subject naming convention across the datasourcex public and internal API, and commit
its ADR. The rule (see docs/adr/0002-subject-path-naming-convention.md, currently authored but
uncommitted): a variable/parameter/property holding a subject's String form is named path; one
typed Subject is named subject. Method names describe an operation and are unaffected.

Rename the String-subject parameters (e.g. in AntPatternNamespace) and the internal
reactive.core / spring.internal occurrences. The one binary-breaking public consequence is
BroadcastEvent's subject: String property — introduce it by deprecate-and-add: add a path
member and keep subject as a @Deprecated alias delegating to it, so the change ships in the 3.x
minor without breaking binary compatibility. The code generator already emits path: String; confirm
it stays that way.

Acceptance criteria

• docs/adr/0002-subject-path-naming-convention.md is committed.
• String-subject parameters/properties are named path; Subject-typed ones are named subject, across public and internal datasourcex code.
• BroadcastEvent exposes path, with subject retained as a @Deprecated alias.
• apiCheck passes; the only public .api diff is the BroadcastEvent deprecate-and-add (no removals).
• ./gradlew check passes.

Blocked by

• Regroup datasourcex reactive modules into reactive/datasource/* #66

[rossdanderson]

Done in commit 97af6c4 on branch sl4jx-consolidation. path/subject naming convention applied across datasourcex (util AntPatternNamespace, reactive.core, spring.internal); BroadcastEvent uses deprecate-and-add (path + @deprecated subject alias); ADR 0002 committed. apiCheck's only diff is the additive getPath(); check green.