fep: unified package integration for FlagOS repositories

[shiptux]

Summary

Adds FEP-19 (Provisional) under sig-os, proposing a unified package integration process for FlagOS repositories: build metadata, package naming, CI artifacts, release publishing to FlagOS Nexus, and shared acceptance checks.

Baseline deliverable per repo is a Debian .deb + RPM .rpm pair published to FlagOS Nexus; Python projects additionally publish a pip-installable wheel on PyPI as a first-class channel. Container images are reserved for builds that need a vendor SDK. Rollout is staged into waves, anchored by the eleven open Wave 1 packaging PRs (FlagCX, FlagTree, FlagGems, FlagScale, FlagQuantum, FlagTensor, FlagAudio, FlagBLAS, FlagDNN, FlagAttention, FlagSparse).

Review routing

sig-os is proposed as a Planned SIG in #35 (charter draft, no Chair yet). Per the bootstrap note in fep/README.md, this FEP is reviewed directly by the TSC until sig-os activates. #35 landing first is preferred but not a hard blocker for review.

What this PR does

• Adds fep/sig-os/19-unified-package-integration.md (Provisional, FEP-19).

Notes for reviewers

• Repository-owning SIGs / WGs co-review package behavior for their modules (table in the FEP, aligned with the Active SIG / Planned SIG / Incubating WG model — wg-ai4s, wg-embodied).
• Non-baseline distributions (Ubuntu 22.04, Debian Trixie, RHEL/openEuler, …) are scoped to the FlagOS SIG set up within each distribution's own community (openKylin, openEuler), not Wave 1.
• Canonical backend suffix list is normalized from FlagCX ADAPTOR_MAP + FlagTree third_party/; aipu removed (no implementation found), FlagCX's du deliberately deferred.
• Status is Provisional; iterating in this PR.