[codex] Scope JetStream dedupe by subject

[SadPossum]

What changed

• derive JetStream Nats-Msg-Id as a fixed 64-character SHA-256 digest of the normalized subject plus canonical outbox GUID
• preserve same-subject/same-ID broker de-duplication while allowing different subjects and event types with the same GUID to coexist in one stream
• add deterministic unit vectors and a real NATS provider scenario covering same-subject replay, same-GUID/different-subject publication, and duplicate-window expiry
• document the rollout overlap and the remaining producer/outbox identity boundary

Root cause and impact

The adapter previously used the GUID alone as Nats-Msg-Id. JetStream de-duplication is stream-wide, so two different subjects sharing an EventId collided even when they represented different public events. This change is transport-internal: it adds no public API or persistence schema.

Module outbox tables are unchanged and remain keyed by GUID alone. Producers must still generate globally collision-resistant integration-event IDs because same-ID rows cannot coexist inside one module outbox even when their subjects differ.

During rollout, an outbox row first published with the old GUID-only key and then retried with the new subject-scoped key can be stored once under each key. Drain the publishing backlog before upgrade when that overlap is unacceptable; consumer inbox idempotency remains required.

Validation

• dotnet restore Gma.Framework.slnx
• dotnet build Gma.Framework.slnx --no-restore -m:1 -nr:false: 0 warnings, 0 errors
• ./eng/test-fast.ps1 -NoBuild: 1,125 passed
• ./eng/test-docker.ps1 -NoBuild: 3 passed, including the real NATS duplicate-window proof
• changed-file dotnet format style and dotnet format analyzers verification
• solution synchronization, repository security, repository release, and git diff --check
• transitive NuGet vulnerability audit: no vulnerable packages

Formatting baseline note

A solution-wide whitespace dotnet format --verify-no-changes is not a valid green signal in this Linux checkout: the repository .gitattributes normalizes tracked C# files to LF while .editorconfig requires CRLF, producing ENDOFLINE diagnostics across untouched baseline files. Changed-file style and analyzer verification passed, and the zero-warning build enforces the configured code-style analyzers.