feat(multiplayer): add reusable room and session package

[liuzqk]

What changed

• adds com.zerogamestudio.zeroengine.multiplayer v0.2.0
• separates platform-neutral session coordination from Steam, FishNet, LocalDirect, and presentation assemblies
• adds room/invite/readiness/retry/reconnect state handling, setup validation, local launcher, sample UI, and EditMode coverage
• keeps FishNet and Steam integrations optional through assembly version defines

Why

GalleryKeeper's hand-written Steam lobby and transport paths treated lobby membership as a ready network session. This package introduces a reusable, fail-closed lifecycle with distinct room, transport, identity, and synchronization phases.

Validation

• Unity 6000.3.10f1 EditMode: 59/59 passed
• Unity 2022.3.62f3 EditMode: 59/59 passed
• Windows development player build passed
• independent LocalDirect host/client processes both reached ready and exited cleanly
• staged package audit: JSON/asmdefs valid, Unity metas complete, GUIDs unique, no project-specific source names

Remaining manual validation

• real Steam invitation, join, disconnect, and reconnect require two Steam accounts after the first game integration

[github-actions]

Unity Test Report

0 tests   0 ✅  0s ⏱️
0 suites  0 💤
0 files    0 ❌

Results for commit 67f9120.

♻️ This comment has been updated with latest results.

[github-actions]

Unity Test Report (Legacy package)

455 tests  +63   455 ✅ +63   0s ⏱️ ±0s
 52 suites + 9     0 💤 ± 0 
  1 files   ± 0     0 ❌ ± 0 

Results for commit 8c7a14c. ± Comparison against base commit e779c09.

[github-actions]

Unity Test Report (Modular packages)

204 tests  +63   204 ✅ +63   0s ⏱️ ±0s
 31 suites + 9     0 💤 ± 0 
  1 files   ± 0     0 ❌ ± 0 

Results for commit 8c7a14c. ± Comparison against base commit e779c09.