fix(engine): align no-preference miner lane-fit score to 0.5

[joaovictor91123]

Summary

• Align computeMinerGoalLaneFit's empty-preferredLabels branch to return 0.5, matching computeLaneFit's documented neutral (never 0 or 1).
• Update engine node:test and vitest coverage that pinned the old 1 behavior.

Closes #8870

Scope

• The PR title follows type(scope): short summary Conventional Commit format, for example fix(api): restore profile access checks.
• This PR is focused and does not mix unrelated backend, UI, MCP, docs, dependency, and deploy changes.
• This follows CONTRIBUTING.md and does not reintroduce GitHub Pages, VitePress, site/, or CNAME.
• I linked a currently open issue this PR resolves (e.g. Closes #123) — a linked open issue is required for every contributor PR.

Validation

• git diff --check
• npm run actionlint
• npm run typecheck
• npm run test:coverage locally; codecov/patch requires ≥99% coverage of the lines AND branches you changed (aim for 100% on your diff so CI variance does not fail near the threshold). Global coverage is a non-blocking trend with a loose 90% backstop, not the gate.
• npm run test:workers
• npm run build:mcp
• npm run test:mcp-pack
• npm run ui:openapi:check
• npm run ui:lint
• npm run ui:typecheck
• npm run ui:build
• npm audit --audit-level=moderate
• New or changed behavior has unit/integration tests for new branches, fallback paths, and sanitizer boundaries

If any required check was skipped, explain why:

• Full suite not re-run locally (Node 24 vs engines Node 22). Diff is a one-line scoring alignment plus updated unit expectations; CI is source of truth.

Safety

• No secrets, wallet details, hotkeys, coldkeys, user PATs, private keys, raw trust scores, private rankings, or private maintainer evidence are exposed.
• Public GitHub text stays sanitized, low-noise, and does not imply compensation guarantees or optimization tactics.
• Auth, cookie, CORS, GitHub App, Cloudflare, or session changes include negative-path tests.
• API/OpenAPI/MCP behavior is updated and tested where needed.
• UI changes use live API data or real empty/error/loading states, not production mock/demo fallbacks.
• Visible UI changes include a UI Evidence section below with JPG/JPEG or PNG screenshots arranged as organized, captioned, clickable thumbnails. SVG screenshots are not used as review evidence. Review-only screenshots or recordings are not committed to the repository.
• Public docs/changelogs are updated where needed; changelogs are only edited for release-prep PRs.

UI Evidence

N/A — no visible UI changes.

Notes

• Switched from fix(db): stale KNOWN_MIGRATION_DUPLICATES entry for migration 90 references a file that no longer exists #8897 retries after fix(db): drop stale migration-90 grandfather entry #8968 died again on the unrelated miner-worktree-allocator-collisions flake.

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[joaovictor91123]

Closing per contributor focus shift away from fix(engine) PRs; picking non-engine issues next.