Move Easy-RSA command logic into PKI

[kemsta]

Summary

• make public *pki.PKI methods the implementation surface for all 34 verified Easy-RSA commands
• replace the five-component constructor boundary with one transactional storage.Backend
• implement writable filesystem/memory backends and read-only legacy behavior, including durable crash recovery, lifecycle/current-state snapshots, confinement, locking, and rollback
• move initialization, lifecycle archives, CRL/DH/PKCS artifact persistence, typed CSR inspection, path-first EKU, DN parsing, and random generation into the root library
• reduce cmd/go-easyrsa to argv/environment parsing, typed option construction, formatting, and exit status

Intentional API changes

• pki.New now accepts a single storage.Backend
• export methods use typed option structs and persist standard artifacts
• command-equivalent revoke methods no longer generate a CRL implicitly; GenCRL remains explicit, matching Easy-RSA
• Expire performs filesystem lifecycle movement; ExpireCert remains the index-only extension

The command matrix remains 34 verified / 10 deferred. This PR adds no new commands.

Verification

• ordinary tests and go vet for all four Go modules on Go 1.25.13 and 1.26.6
• root and CLI Easy-RSA v3.2.6 E2E suites
• root storage/PKI and CLI race suites
• golangci-lint v2.12.2 for root and CLI
• govulncheck v1.7.0 for all four modules
• actionlint
• Linux, Windows, and AIX compilation checks
• tidy, module verification, and no-replace checks
• external go install at the final branch commit plus lifecycle smoke test

No tags or releases are included.