docs(security): migrate secure boot to RSA-3072 and reorder lockdown phases

[odudex]

Documentation-only update to the security roadmap (docs/security-plan.md, docs/secure-boot.md) after reviewing the ESP-IDF v6.0.2 security changes.

Highlights

• RSA-3072 instead of ECDSA-P256 — ECDSA Secure Boot v2 is non-functional on ESP32-P4 silicon (chip errata; would need CONFIG_SECURE_BOOT_INSECURE). RSA-3072 is the only working scheme and the faster verifier here. Fixes the algorithm table, sdkconfig option names, and updates tooling to esptool v5.
• Three signing keys (two rotations) — uses all three digest slots (KEY0–2); the flash-encryption key moves to the Key Manager / KEY3.
• Lockdown phases reordered to match the safe eFuse burn order — every read-protected key is burned before secure boot write-protects RD_DIS: NVS encryption → flash encryption → secure boot → SD updates → release lockdown.
• NVS encryption coupled to PIN setup (new Phase 3) — KEY4 burn + NVS encryption happen while setting the PIN, so the PIN hash lands directly in encrypted storage: at-rest protection by default, independent of flash encryption.
• Deployment profiles — Profile A (secure boot + NVS encryption, fully on-device) vs. Profile B (adds flash encryption + release lockdown, serial).
• Housekeeping — dual-OTA partition table marked adopted; six never-used placeholder key files removed; kern_secure_boot/README.md refreshed.

No code changes.