ci: harden Linux dependency fetch resilience

[blakinio]

Objective

Finish OTC-20260816-linux-ci-hybrid post-merge validation without re-enabling Windows.

Evidence driving this follow-up

PR #331 is already merged and its exact-head CI run 31935503532 passed both hosted Linux matrix jobs, the real release-artifact Xvfb startup smoke, and CI / Required.

The first compile-scope push on main (31937211914, merge SHA c4b1919e16fb2931c74f32cb310229703dbf893c) failed during vcpkg dependency installation because the freetype 2.14.3 source endpoint returned HTTP 504 through all three built-in download attempts. Setup, workflow validation and earlier dependencies succeeded; this was not an OTClient compile diagnostic. A same-SHA rerun reached both Run CMake jobs but was intentionally cancelled when newer documentation-only main commits superseded the old run via the repository concurrency policy.

Changes

• archive stale OTC-20260712-client-test-foundation ownership now that PR test: establish client unit and integration test foundation #3 is merged;
• allow the current task to own .github/workflows/reusable-build-linux.yml;
• cache only vcpkg/downloads using pinned actions/cache v6.1.0;
• keep the existing CMake action as the primary attempt;
• if that attempt fails, retry the same pinned configure/build action exactly once;
• preserve all existing Linux tests, artifact upload, headless startup smoke and CI / Required semantics.

Failure semantics

The first CMake attempt uses continue-on-error only to permit the bounded second attempt. A second failure remains a normal job failure, so deterministic code/configuration failures are not converted to success.

Boundaries

• Windows general CI remains disabled.
• No Synology/Track A runtime workflow is changed.
• No production source, runtime asset, protocol, repository setting or physical gameplay behavior is changed.
• vcpkg/downloads contains source/tool downloads, not OTClient runtime state or built client artifacts.

Validation contract

Before merge require on the exact PR head:

• workflow/actionlint validation;
• Linux release build;
• Linux tests build plus unit/Lua/integration tests;
• real release-artifact Client Startup Smoke - Linux under Xvfb;
• CI / Required success.

After merge require a fresh compile-scope main run with the same Linux build/smoke/Required chain. Only then archive OTC-20260816-linux-ci-hybrid and release ownership.

Primary implementation: #331.
Task: OTC-20260816-linux-ci-hybrid.

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[blakinio]

Closing this follow-up as obsolete for OTC-20260816-linux-ci-hybrid.

Fresh closeout evidence resolved the post-merge ambiguity without requiring a workflow behavior change:

• primary PR ci: move required builds to Linux and preserve runtime hybrid #331 exact head 4c50f1d5843bfe067cca19519e25e4fa9dc7ccfe passed both Linux builds, unit/Lua/integration tests, real release-artifact Xvfb startup smoke and CI / Required in run 31935503532;
• that exact head and squash merge c4b1919e16fb2931c74f32cb310229703dbf893c have the identical tree 6a51b91962c61b72a97a308d07292779ac7c3407;
• the failed post-merge attempt is directly attributable to repeated HTTP 504 responses while vcpkg fetched freetype from gitlab.freedesktop.org, not an OTClient compile diagnostic;
• the task was terminally archived through docs(ci): archive Linux CI hybrid task #341 and current main CI validates the docs-only path scope.

The proposed vcpkg/downloads cache + bounded CMake retry may be considered later as a separate CI-resilience optimization, but it should not reopen this completed hybrid-routing task or bundle the unrelated stale OTC-20260712-client-test-foundation cleanup. No review thread or owner decision requires this PR to remain open.