sharing 1/7: Transfer library (enums, comms, serialization, test infra)

[keithharvey]

📚 Stacked split of #5704 — review bottom-up

• 1/7 · Transfer library (enums, comms, serialization, test infra) (sharing 1/7: Transfer library (enums, comms, serialization, test infra) #8125) ← you are here
• 2/7 · Modes & economy boundary (presets, waterfill, share ledger) (sharing 2/7: Modes & economy boundary (presets, waterfill, share ledger) #8126)
• 3/7 · Policies (context → policy → policy_result → action) (sharing 3/7: Policies (context → policy → policy_result → action) #8123)
• 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; together they reproduce the sharing_tab branch (bar one intentional change — index.lua lazy-loads the mode helpers).

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Stack 1 of 7 splitting #5704. The leaf layer everything builds on — pure plumbing, no policy/modes/economy logic.

• team_transfer/* — transfer_enums, sharing_mode_enums, 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
• test infrastructure — spec builders + spec_helper. index.lua now lazy-loads the mode helpers (which pull the pipeline), so the lower layers stay pipeline-free. Includes the policy_events / unit_sharing_categories specs.

Compiles and tests standalone.

[github-actions]

Integration Test Results

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

Results for commit 9ce16f6. ± Comparison against base commit d4e6e02.