Continue Steam SDK 1.65 API migration

[ywgATustcbbs]

This is a follow-up to #1088 and is intentionally based on its head branch.

Steam SDK 1.65 migration update

Progress

• The follow-up implementation is consolidated into one commit on top of the PR head base 574e27fbf3dd228e347d21e60ae430867f0cee3c: 6742f4d1d6300ea98ea71111a3372019ca80659e.
• The original PR branch could not be updated directly because upstream rejected the push with HTTP 403. This follow-up uses my fork branch task/steam-sdk-pr1088.
• Phase 4: 46/46 packages reached final independent PASS.
• Phase 5: one canonical solution-level Release|Win32 Rebuild completed with exit code 0 and 0 errors. The new SDK #else path and the x86 Steam SDK 1.65 steam_api.lib link were verified. The deployable build outputs are exactly client.dll, server.dll, and missionchooser.dll.
• Warning closure: 88,542 total warnings across 13 codes; 88,416 were policy-ignored; 126 retained events at 121 locations across 10 codes. All retained warnings were legacy or external-SDK warnings; migration-attributable/actionable warnings: 0. No warning-driven source fixes were needed.

Change classification

The following ledgers overlap and are not additive. They should not be interpreted as a three-way file/line/code-block count, and there is no trustworthy non-overlapping count for “rewritten compatibility logic” versus “simple call fixes” versus “verbatim copies”.

• 25 semantic compatibility/design-change records (A=6, B=19). This is a design-matrix count, not a count of files, lines, or code blocks.
• 9 API branch substitutions already present in PR compatibilty for steam interface v9 #1088: 3 RequestCurrentStats, 5 user RequestUserStats, and 1 dedicated-server RequestUserStats. These remain covered by callback/pump/teardown review and are not claimed as fully accepted simple fixes.
• 1,086 new defensive wrapper units for business Steam calls, covering both changed and unchanged calls. The unchanged-only subset was not separately aggregated, so no trustworthy unchanged-only number is claimed.
• 4 pre-existing guarded units (CB-01 through CB-04) retained from the PR structure.
• Total unique defensive-wrapper scope: 1,090 = 1,086 + 4. This wrapper ledger is not an additional rewrite count.

Key user decisions

• Preserve the PR's old/new conditional structure.
• Guard every in-scope business Steam accessor, method, global function, and callback registration explicitly with the STEAMAPPS_INTERFACE_VERSION008 old/new branches, including calls whose API did not change.
• Preserve the macro expression as-is. Its origin was intentionally not investigated; no extra version detection, fallback, or second ABI was introduced.
• Build and link only the new path. The old path is mirror/static-review-only and remains Not built by user decision.
• Treat the patched engine.dll and external steam_api.dll as runtime inputs. Their provenance, ABI, and package relationship are deferred to runtime validation.
• The build result is one unified Release gate, not nine independent builds.

Manual smoke testing

Verified by user:

• Game starts normally.
• In-game inventory works.
• A self-hosted lobby can be created and played for approximately 30 seconds without an observed problem.
• Leaderboards are visible.
• Lobby list is visible.

Not yet verified:

• Long-duration stability, a full level, and repeated map transitions.
• Joining another player's lobby, invitations, leaving, and reconnecting.
• Dedicated-server operation.
• Inventory asynchronous completion, cancellation, error handling, and map-end behavior.
• Full stats, achievements, and player-reporting callback flows.
• Workshop/UGC, friends/presence, input, HTTP, and Steam Deck/Proton coverage.
• Clean shutdown/reinitialization and failure paths.
• Runtime logs/crash dumps and engine.dll/steam_api.dll provenance and ABI validation.

Phase 6 therefore has partial smoke evidence, but is not a complete runtime PASS yet.

[ywgATustcbbs]

Most of the missing tests are long term tests / tests that I can't perform by myself. So I mark this pr as ready for review.