test: add process-level PTY backup restore acceptance (M6.5.2)

[KeyffMS]

Implements M6.5.2 from roadmap #26: process-level PTY/RTU acceptance for the production Backup / Diff / Restore path added in #77.

Scope:

• extends the existing connection_process_acceptance --write-fixture harness instead of adding a second PTY framework;
• runs the real vfd-lantern TUI process with isolated HOME/XDG state;
• captures a complete source backup through the product UI;
• loads the persisted source backup in a new verified session;
• prepares a fresh pre-restore backup and semantic restore plan;
• verifies wrong exact confirmation performs zero writes;
• executes a deterministic two-step Normal restore over write-capable conformance PTY/RTU and asserts exactly two ordered FC06 writes;
• verifies durable operation/write audit evidence;
• injects a device exception on restore step 2 and proves terminal abort with no retry/catch-up write.

The fixture extends the fictional example profile with one additional Normal writable parameter only inside the process harness; no production profile or safety policy is changed.

Refs #25
Refs #26