ci: republish the coverage report to GitHub Pages

[ggreif]

#6136 dropped the report-site pipeline, but its final deploy is still live: https://caffeinelabs.github.io/motoko/ serves a page frozen at 2026-05-21 whose every link 404s. Two independent causes, both verified:

• the old index was generated by tree -H . --fromfile, which emits ../-prefixed hrefs — served from /motoko/, ../coverage/ resolves to caffeinelabs.github.io/coverage/, outside the Pages prefix (404). Reproduced locally with the exact command.
• Pages serves a directory URL only when that directory has an index.html and never generates listings, so e.g. /coverage/ir_passes/ 404s while /coverage/ir_passes/tailcall.ml.html is fine.

This restores the pipeline for coverage only — the old site's base-doc and docs entries are gone — with a hand-written landing page whose single link is relative, so it works at any prefix. No tree dependency.

report-site deliberately lives in packages, not common-constituents: the latter feeds the *-systems-go aggregates and would pull the instrumented coverage build into every test job.

Verified locally: the derivation builds against a stub coverage input and emits href="./coverage/"; nix eval .#report-site.name resolves; actionlint reports nothing on the new job (the findings it does report are pre-existing, in other jobs).

Note the reports job only runs on push to master, so it cannot exercise itself on this PR — the first real deploy happens on merge. Existing /coverage/... URLs keep working, since the report stays under coverage/.

No Changelog entry: this is CI/infra, with no user-visible compiler change.

[github-actions]

Comparing from 9cf37c3 to ce6e061:
The produced WebAssembly code seems to be completely unchanged.
In terms of gas, no changes are observed in 5 tests.
In terms of size, no changes are observed in 5 tests.

[github-actions]

Cursor AI review

👍 APPROVE — looks safe to merge

Category	Assessment	Details
Summary	✅	Restores a master-only GitHub Pages deploy of the bisect coverage report via a new report-site Nix package and reports job, replacing the broken tree-generated index with a relative-link landing page.
Code Quality	✅	Small, focused restore: reuses existing tests.coverage, keeps report-site out of common-constituents (avoids dragging coverage into *-systems-go), no speculative extras.
Consistency	✅	Matches artifacts job patterns (pinned action SHAs, cachix/install-nix-action + cachix-action, cp -rL for symlink trees) and existing contents: write workflow perms for gh-pages push.
Correctness	✅	Traced flake.nix → report-site.nix → tests.coverage (nix/tests.nix installPhase / test/Makefile bisect-ppx-report … -o coverage); landing href ./coverage/ stays under /motoko/; job is push+master only with SHA-pinned deploy action; no trigger/permission broadening.
Tests	✅	No Motoko .mo/.ok changes; deploy is intentionally master-only (cannot self-exercise on the PR). Coverage artifact already produced by common-tests via filterTests "common".
Changelog	✅	CI/infra only; correctly omitted (no user-visible moc/language change).

Verdict

Decision: APPROVE
Risk: Low
Reason: Contained CI restore with master-only deploy, pinned actions, and no compiler/RTS/typing surface. Relative index link and package placement match the stated failure mode and existing flake/CI structure.

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Generated for commit ce6e061

[caffeine-ci-bot]

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