sharing 6/7: Sharing Tab UI

[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)
• 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) ← you are here
• 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 6 of 7 splitting #5704. Builds on 5/7.

The player-facing tab and supporting widgets:

• gui_advplayerslist sharing tab + modules, plus team_transfer/gui_advplayerlist/* policy/resource/validation + unsynced comms helpers
• gui_top_bar share/conversion areas, gui_chat, cmd_share_unit, cmd_take widget, selectedunits + DrawPrimitiveAtUnit shaders
• interface.json / tips.json strings

[github-actions]

Integration Test Results

18 tests  ±0   9 ✅ +2   20s ⏱️ +12s
 1 suites ±0   8 💤 ±0 
 1 files   ±0   1 ❌  - 2 

For more details on these failures, see this check.

Results for commit 5ec9c9b. ± Comparison against base commit b77838c.

♻️ This comment has been updated with latest results.