sharing 2/7: Transfer library (comms, serialization, types, test builders)

[keithharvey]

📚 Stacked split of #5704 — review bottom-up

• 1/7 · Policies (context → policy → policy_result → action) (sharing 3/7: Policies (context → policy → policy_result → action) #8123)
• 2/7 · Transfer library (comms, serialization, types, test builders) (sharing 2/7: Transfer library (comms, serialization, types, test builders) #8061) ← you are here
• 3/7 · Modes & economy boundary (presets, waterfill, share ledger) (sharing 3/7: Modes & economy boundary (presets, waterfill, share ledger) #8124)
• 4/7 · Transfer runtime (controllers, economy boundary, gadget swap) (sharing 4/7: Transfer runtime (controllers, economy boundary, gadget swap) #8062)
• 5/7 · Tech Core / Keystone (sharing 5/7: Tech Core / Keystone #8063)
• 6/7 · Sharing Tab UI (sharing 6/7: Sharing Tab UI #8064)
• 7/7 · Game modes export (widget, headless startscript, CI workflow) (sharing 7/7: Game modes export (widget, headless startscript, CI workflow) #8095)

Each PR merges into the one below it; all seven squash-equal the sharing_tab branch (verified byte-identical).

---

Stack 2 of 7 splitting #5704. Builds on 1/7.

The shared transfer plumbing the policy pipeline and the runtime/UI build on — no modes or economy yet (those are 3/7):

• team_transfer/* — comms (resource / unit / take / tech_blocking), serialization_helpers, team_resource_data, unit_sharing_categories, tech_blocking_shared, policy_events
• messaging + i18n — lua_rules_msg, game_message, i18n interpolate; types/Spring.lua
• shared test infrastructure — spec builders + spec_helper, and the team_transfer specs

[github-actions]

Integration Test Results

14 tests   6 ✅  3s ⏱️
 1 suites  8 💤
 1 files    0 ❌

Results for commit 4ad5fe3.

♻️ This comment has been updated with latest results.

[keithharvey]

Not actually merged — GitHub auto-closed this as "merged" when a base-branch force-push made the base contain this branch. Reordered + replaced by #8125 (Transfer library, 1/7).