refactor(tui): extract _SlashWorker client + session db into tui_gateway modules (server.py god-file slice R1)

[andrexibiza]

Summary

tui_gateway/server.py god-file slice R1: extract the _SlashWorker subprocess client + session-db cluster from tui_gateway/server.py (13,908 lines) into tui_gateway/slash_worker_client.py and tui_gateway/session_db.py. Part of the repo-wide large-file decomposition (tracker #78647).

What changed and why

• _SlashWorker + _SLASH_WORKER_TIMEOUT_S (window 158–162 + 326–459, golden sha e71683bf…) → tui_gateway/slash_worker_client.py — zero forward deps, git_probe.py extraction is the in-tree precedent
• Session DB cluster (window 1301–1508, golden sha cea6186d…, 12 members) → tui_gateway/session_db.py — 9/12 members have zero outside refs; the 2 global state lines stay on server.py (test-pinned)
• server.py re-exports all moved names identity-preserving (seam: server.X is module.X)
• Double-blind: 2 blind region analysts (pass A + pass B) → consensus adjudication (R1-CONSENSUS.md) → implementer → 2 blind re-reviewers (both APPROVED)

Testing

• 63 tests pass (5 dedicated slash-worker files + 2 new extraction tests) + 523 tui server/ws tests
• Full tests/tui_gateway/: 391 passed, 4 failed — identical 4 proven at the pristine pin (timing flakes + Windows SIGPIPE)
• Seam identity 14/14 re-export names verified is-identical
• Golden shas verified against the consensus contract
• ruff clean · git diff --check clean · LF-only · DCO signed · Windows-tested

Coordination / interlock

• Epic: Part of [COMPLETE] Large-file decomposition: 20/20 done #78647 (Shard all 20 god files) · Target issue: Part of Shard tui_gateway/server.py (god-file decomposition) #78630
• Sibling PRs: the 4 other tui_gateway/server.py slices (change-watcher, pet-payload, config.set, projects) — disjoint clusters, same wave
• Dedup: no existing PR extracts these clusters (verified via open-PR scan)
• Credit: extraction pattern follows the web_server wave (refactor(web): extract model-assignment helpers into web_server_model_assignment (web_server.py god-file slice R1-C9) #79123–refactor(web): extract WhatsApp onboarding into web_routers/whatsapp_onboarding (web_server.py god-file slice R4-C3) #79129) and the run.py Extract Gateway Platform Routing from gateway/run.py #54962 precedent; seam = the method_ctx.HandlerRegistry globals-rebinding contract

Part of #78647
Part of #78630

[andrexibiza]

🔒 This shard is part of the tui_gateway/server.py KILL LOCK — the permanent record of the 13,908-line whole, the mess it caused (#60654 WS-frame stalls/frozen desktop, #60800 cold-start stalls, #33503 shell injection, #76285 board-slug validation, #76701 SessionDB abandonment, FIPS crashes), every shard in the wave, and every open fixer PR still fighting the surface (#72370 #76366 #67628 #58339 #77391 #66874 #77745 #72231 #62598 #33503 #74408 #74350 #65192 #77860 #76298 #78036 #74701 #72295 #73800 #76131 #68468 #51596 #53373 #76701 #70226).

Lock: posted on #78630 · Indexed by the Kill All Gods meta-issue #78647.

[andrexibiza]

Pushed 52470ebc54: the extraction contract no longer uses hardcoded C:/tmp, which does not exist on Linux and therefore made _configured_cwd_from_cfg correctly return None. The test now creates a real tmp_path directory. Verification: session-db extraction, slash-worker extraction, and /undo suites pass 54/54 on Windows and 54/54 under Ubuntu/WSL.