refactor(binding): add process-neutral value conversion

[jmgilman]

Summary

• validate and bind process-neutral capability input shapes
• re-bind worker-decoded arguments into exact registered Go inputs and fresh authorization maps
• centralize bounded Starlark/value conversion while preserving numeric identity and PR fix(binding): reject oversized final-value allocations #26 allocation defenses
• retain existing execution callers for the first increment of the worker-isolation cutover

Review

One persistent reviewer completed three bounded passes. All blocking findings were fixed; the final pass reported zero merge blockers.

Verification

• mise exec -- golangci-lint fmt --config .golangci.yml
• mise exec -- go test ./internal/binding -count=1
• mise exec -- go test ./internal/execution -run '^TestExecute' -count=1

Part of the worker-isolation remediation for #12; does not close the issue.