feat(daemon): continuously pump session output

[msmps]

Summary

• drain PTY output continuously from session start
• centralize observed terminal state, revisions, process exit, and bounded finalization
• wake snapshot and wait requests from session observations instead of polling
• preserve final output when the PTY closes, including descriptor-inheriting descendants
• document finalization and output-complete terminology

Verification

• cargo fmt --all
• cargo clippy --all --benches --tests --examples --all-features -- -D warnings
• cargo test --all (187 passed, 1 intentionally ignored)
• git diff --check HEAD^ HEAD

Story

This is the first revival slice. It makes session output ownership reliable enough for subsequent recording and asciicast work to build on a continuously observed terminal stream.