Accept alpha.8 isolated execution

[R055LE]

What changed

• record the permanent alpha.8 artifact and owner-device isolated-execution acceptance
• make alpha.8 the current accepted tester build across the README, status, history, and signing guide
• add the transcript/IME viewport bug and missing saved-history execution-mode label to the backlog

Why

The merged isolated-execution implementation had passed automated coverage but still needed signed physical-device evidence. The owner-device fixture run now proves strict failure containment, child-state isolation, and persistent-shell recovery without reconnecting.

The same run exposed two separate presentation problems. The software keyboard can strand useful transcript content above the viewport, and the saved-history formatter omits the isolated label even though Room retains the mode.

Impact

Alpha.8 becomes the accepted tester build. No application code changes in this PR. The two UX findings remain explicitly deferred with expected behavior recorded for later fixes.

Validation

• exact merged-main Android CI candidate passed
• signed APK checksum, package/version, permanent certificate, v2/v3 signatures, and 16 KiB alignment verified
• Galaxy S25 Ultra fixture run proved isolated exit 1, unreachable-line suppression, state containment, saved transcript recovery, and successful persistent follow-up
• documentation diff and private-material scans passed
• disposable fixture and temporary tailnet forward stopped after testing