fix(update): restore legacy self-update compatibility

[ZhiXiao-Lin]

Summary

• add a 16 KiB fail-closed release marker that satisfies the archive contract hard-coded in A3S 0.9.9 through 0.10.10
• keep Anthropic SRT runtime code and dependencies removed
• package and verify the marker in every tar archive and the Windows zip
• add regression tests for the legacy JSON contract, inert CLI, executable mode, and release workflow

Root cause

A3S 0.10.8 validates support/managed-srt before replacing its own binary. Releases from 0.10.11 onward removed that tree together with SRT, so the old process rejects the otherwise valid archive before it can install the SRT-free binary.

Validation

• cargo test --locked --test legacy_self_update_bridge
• public v0.10.8 Apple Silicon binary completed a controlled self-update through the new bridge: 0.10.8 -> 9.9.9
• installed bridge contained only four files; its CLI exited 1 with the retirement message
• release workflow YAML parsed successfully and git diff --check passed