ci: move required builds to Linux and preserve runtime hybrid

[blakinio]

Objective

Make ordinary OTClient CI Linux-only on GitHub-hosted runners, including a bounded headless startup smoke of the built Linux client, while preserving Synology/self-hosted capacity for specialized real runtime/LAN/display/input/persistent-session work.

Changes

• replace the required Windows build gate with the existing reusable-build-linux.yml matrix;
• remove reusable-build-windows.yml;
• add a required GitHub-hosted Client Startup Smoke - Linux job after the Linux build;
• download the real linux-linux-release artifact, verify shared-library resolution, and start otclient under Xvfb with software GL, null OpenAL output, and an isolated --user-dir;
• require the client to remain alive for a bounded 20-second startup window and upload dependency/startup logs as evidence;
• carry forward safe path-scope optimization so docs/task-only PRs do not allocate unrelated Fast Checks/Lua/build/smoke runners;
• make pull_request.closed create a no-work run in the same CI concurrency group, so a closed/superseded PR cancels any older in-progress build instead of leaving orphaned runner demand;
• stop automatically retrying intentionally cancelled superseded CI runs;
• leave dedicated Synology/runtime workflow files untouched.

Runner boundary

GitHub-hosted: static/workflow/Lua validation, unit/integration tests, Linux build, artifact dependency validation, bounded headless startup smoke.

Synology/self-hosted: persistent OTClient session, real display/input ownership, login/walking/clicking, LAN/runtime integration, long-lived observation and direct physical gameplay evidence.

The GitHub headless smoke must not be treated as a replacement for Synology physical runtime E2E.

Coordination

• supersedes closed PR ci: reduce hosted-runner queue amplification #328, which retained the Windows gate;
• live validation found PR ci: reduce hosted-runner queue amplification #328's already-started Windows CI still consuming hosted runners after closure; this PR adds the prospective closed-PR cancellation invariant;
• PR ci: add dedicated OTClient and OTS runner stack #280 remains a separate specialized Synology/runtime lane and is intentionally not modified here;
• historical client-test-foundation PR test: establish client unit and integration test foundation #3 is merged, but its stale active task still lists reusable-build-linux.yml as owned, so this PR deliberately adds the smoke in ci.yml without modifying that reusable workflow.

Validation contract

• workflow syntax/actionlint must pass;
• both Linux build matrix jobs must pass on GitHub-hosted Ubuntu;
• the exact-head startup-smoke job must execute the real release artifact and survive the bounded virtual-display startup window;
• CI / Required must require both build and startup smoke when compile scope is true;
• no Windows build job may be emitted;
• closed-PR events must skip ordinary jobs while sharing the PR concurrency key;
• Synology runtime workflows remain outside this changed-file set.

The pinned vcpkg baseline's x64-linux triplet links vcpkg libraries statically, so the downloaded release artifact does not require a second vcpkg installation on the smoke runner; only system X/GL/OpenAL runtime packages are installed there.

Task: OTC-20260816-linux-ci-hybrid.

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[blakinio]

Fresh exact-head audit on db6e4357e1d51d112c306248fcdebd5ef1127397:

• changed-file set is limited to .github/workflows/ci.yml, .github/workflows/infrastructure-retry.yml, removal of .github/workflows/reusable-build-windows.yml, and the task record;
• general CI now references reusable-build-linux.yml; the accepted Actions graph references checks, Lua, and Linux reusable workflows only;
• reusable-build-linux.yml uses GitHub-hosted ubuntu-24.04 with linux-tests and linux-release matrix entries;
• exact-head observed jobs use runner_group_name: GitHub Actions; Linux build jobs are running in parallel on ubuntu-24.04;
• PR ci: add dedicated OTClient and OTS runner stack #280 Synology/runtime paths are not in this diff; ci: reduce hosted-runner queue amplification #328 is intentionally closed superseded;
• yamllint, actionlint, XML validation, Lua syntax, and informational static analysis have passed so far.

No material audit finding identified. Final acceptance still requires both Linux matrix jobs and CI / Required to succeed on this exact head before merge.