sharing 3/4: Tech Core / Keystone

[keithharvey]

📚 Stacked split of #5704 — review bottom-up

• 1/4 · Foundations (modes, economy boundary, transfer library) (sharing 1/4: foundations (modes, economy boundary, transfer library) #7962)
• 2/4 · Transfer runtime (controllers, economy boundary, gadget swap) (sharing 2/4: transfer runtime (controllers, economy boundary, gadget swap) #7963)
• 3/4 · Tech Core / Keystone (sharing 3/4: Tech Core / Keystone #7964) ← you are here
• 4/4 · Sharing Tab UI (sharing 4/4: Sharing Tab UI #7965)

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

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Stack 3 of 4 splitting #5704. Builds on 2/4.

Self-contained tech-tier feature on top of the transfer foundation:

• add Arm/Cor/Legion Keystone buildings (reclaimable, drive tech level)
• rework game_tech_blocking (revert tech level when Keystones are destroyed)
• modes/sharing/tech_core preset + spec + in-game test widget
• alldefs_post, units.json, gui_tech_points (lua/rcss/rml)

[github-actions]

Integration Test Results

19 tests  +1   8 ✅ +1   8s ⏱️ +2s
 1 suites ±0   8 💤 ±0 
 1 files   ±0   3 ❌ ±0 

For more details on these failures, see this check.

Results for commit 0e33c8a. ± Comparison against base commit 7f9a08f.

♻️ This comment has been updated with latest results.