chore: migrate release pipeline to signed filemap and wire rainyun s3

.trae/specs/pdc-incremental-migration/spec.md

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+# PDC Incremental Update Migration
+
+## Goal
+
+Replace VeloPack-based incremental packaging with a unified signed FileMap pipeline and prepare for PDC/S3 distribution compatibility, while keeping Launcher installation, rollback, and update orchestration ownership unchanged.
+
+## Stage 1 (Completed in this round)
+
+- Release workflow outputs signed FileMap incremental assets as the primary path:
+  - `files-windows-x64.json` / `.sig` / `update-windows-x64.zip`
+  - `files-windows-x86.json` / `.sig` / `update-windows-x86.zip`
+  - `files-linux-x64.json` / `.sig` / `update-linux-x64.zip`
+- Launcher and host update runtime remove VeloPack branches and return to signed FileMap apply path.
+- Host update asset discovery supports platform-scoped names with fallback to legacy generic names.
+- Optional S3 sync publishes incremental assets in parallel with GitHub Release assets.
+
+## Stage 2 (In Progress)
+
+- Introduce PDC-compatible update source (`pdc`) with fallback to GitHub.
+- Add PDC metadata/latest/distribution API consumption abstraction.
+- Keep Launcher install/apply/rollback state machine unchanged.
+- Prepare `phainon.yml`-compatible release metadata for future PDCC integration.
+
+## Acceptance
+
+- `release.yml` no longer contains VeloPack packaging steps.
+- Windows x64/x86 and Linux x64 release jobs all upload signed FileMap incremental assets.
+- Host auto-update can detect and download platform-matching signed FileMap assets.
+- Launcher `update apply` succeeds with signed FileMap payload and rollback behavior remains unchanged.
+- Optional S3 upload step works when S3 secrets/vars are configured.