refactor(tui): move config.set handler into methods_config (server.py god-file slice R4)

[andrexibiza]

Summary

tui_gateway/server.py god-file slice R4: move the config.set handler from tui_gateway/server.py into the existing split module tui_gateway/methods_config.py. Part of the repo-wide large-file decomposition (tracker #78647).

What changed and why

• config.set handler (window 10471–11162, golden sha 0788df80…) → methods_config.py (existing split module with the HandlerRegistry.install globals-rebinding contract)
• Both stale NOTE copies deleted — the duplicated move-directive at server.py 10468–10470 and methods_config.py docstring 3–6 (the NOTE said "touches its body" — the code had already moved; the NOTE was stale)
• server.py re-exports the moved names identity-preserving
• PR-collision resolved by the consensus lane: refactor: single-owner model switch parsing + effective-model resolution #74229 (model-switch parsing) merged 2026-07-29 — its changes are already inside the window at the pin; region verified unclaimed by open PRs
• TUI gateway: WebSocket frame stalls >10s causing desktop freeze and "request timed out" toasts #60654 lock/emit semantics byte-identical (the WS-stall plague class constraint)
• Double-blind: 2 analysts → consensus (PR-collision adjudicated with evidence) → implementer → 2 blind re-reviewers (both APPROVED)

Testing

• Config-related tests pass before/after (baseline parity)
• New seam-identity + aggressive tests (valid/invalid values, profile scope, effective-model resolution edges)
• TUI gateway: WebSocket frame stalls >10s causing desktop freeze and "request timed out" toasts #60654 semantics verified byte-identical
• Golden sha verified at pin
• ruff clean · git diff --check clean · LF-only · DCO signed

Coordination / interlock

• Epic: Part of [COMPLETE] Large-file decomposition: 20/20 done #78647 · Target issue: Part of Shard tui_gateway/server.py (god-file decomposition) #78630
• Sibling PRs: the 4 other tui slices — disjoint clusters
• Dedup: no existing PR moves config.set (open PRs fix(tui): close profile-owned SessionDB handles on teardown #78970/fix(tui-gateway): retry and log unflushed-message persist failures before teardown discards them #79109 touch only R1 teardown hunks)
• Credit: seam = HandlerRegistry globals-rebinding contract; the in-repo move directive made this the natural first slice

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.