feat(session): preserve finalized session evidence

[msmps]

Summary

• retain bounded in-memory tombstones after natural exit and explicit kill
• preserve exit metadata, launch settings, final full screen, output completeness, and the last 64 KiB of raw output
• keep tombstones separate from live sessions with a 100-entry cap and 10-minute TTL
• allow snapshot and logs to recover final evidence while live-only commands return SESSION_EXITED
• preserve legacy protocol compatibility with a version-gated error fallback
• document the exited-session evidence window

Lifecycle guarantees

• natural exit transitions atomically from live discovery to finalized evidence
• tombstones do not count toward live-session limits or daemon idle activity
• live names resolve before tombstoned names and tombstoned names may be reused
• expired, evicted, unknown, and daemon-restarted sessions receive honest recovery guidance

Verification

• cargo build
• cargo build --release
• cargo test --all
• cargo clippy --all --benches --tests --examples --all-features -- -D warnings
• cargo fmt --all -- --check
• isolated manual validation recovered final screen and raw output after exit code 7, then confirmed input returned SESSION_EXITED