Expose generation-bound PTY activity status

[myobie]

What changed

• add a single generation-bound live activity lease with ordered unknown, active, child_command, and idle states
• expose activity through STATUS, queryStats(), pty stats, and a public connectActivityPublisher() API
• reset activity to unknown on publisher loss, daemon replacement, malformed owner commands, stale epochs, or skipped sequences
• expose modes.alternateScreen as a diagnostic without using terminal modes as semantic idle authority
• add stable harness-neutral fixtures and adversarial lease/protocol/integration coverage

Why

DING/provider adapters need an explicit live activity fact. Screen text and terminal modes are stale or ambiguous, so PTY must carry only generic publisher state and keep provider-specific hook/log interpretation outside this repository.

Closes #129. Implements the alternateScreen STATUS diagnostic portion of #130.

Validation

• normal merge of current main 025a9034ab0aa73a77ef2fe7f0ab9123ea170396
• npm run build
• npm run typecheck
• activity/protocol/integration/stats contract: 107 passed
• current-main geometry/stream/recovery/locking bundle: 111 passed
• npm run verify-docs: 13 examples passed; 2 VRS documents and 11 requirement IDs verified
• full local suite: 1,578 passed, 21 skipped; only the two pre-existing host Nano cases failed with Incomplete terminfo entry
• exact-head hosted Build run 30845266651: passed
• exact-head hosted Vitest run 30845266695: passed

Draft only. No merge or deployment.