chore: promote surfaces/workers/_shared to workspace package

[jithinraj]

Context

PR #353 (Vitest v4) added @peac/contracts and @peac/mappings-tap as root devDependencies to work around vitest v4's stricter module resolution. The surfaces/workers/_shared/core/ directory imports these packages via bare specifiers but is not itself a workspace package, so pnpm's strict isolation prevents resolution from that location.

The root devDep workaround anchors resolution via root node_modules, which works but is fragile -- someone could "clean up" these seemingly unnecessary root devDeps and re-break CI.

Proposed fix

Move surfaces/workers/_shared into a real workspace package (e.g. @peac/worker-shared) or merge it into the existing packages/worker-core. This restores "declared deps only" semantics and removes the need for root devDep workarounds.

Option A: New workspace package

• Create packages/worker-shared/package.json with proper dependencies on @peac/contracts and @peac/mappings-tap
• Move surfaces/workers/_shared/core/ contents there
• Update imports in worker packages (akamai, fastly, cloudflare)
• Remove root devDeps for @peac/contracts and @peac/mappings-tap

Option B: Merge into worker-core

• Move shared types, errors, and verification logic from _shared/core/ into packages/worker-core
• Update imports in worker packages
• Remove root devDeps

Notes

• Both @peac/contracts and @peac/mappings-tap are needed (not just contracts)
• Worker tests (pnpm test:core) exercise these imports and will catch regressions
• The deprecated surfaces/_shared/contracts/ re-export layer should also be cleaned up