Add a non-interactive release-bundle installer upgrade mode

[Staphylococcus]

Parent: #91

Depends on: #95, #100

Outcome

Allow a verified release bundle to update a host that passes the runtime compatibility checks without rerunning interactive setup or TV pairing.

Scope

• Add an explicit install.sh --upgrade mode.
• Expose a candidate-specific non-mutating preflight that checks the candidate installer requirements defined with Add runtime host upgrade compatibility preflight #100.
• Refuse an incompatible host before privileged mutation.
• Load and preserve the existing config path, settings, updates.channel, and profile-scoped native credentials.
• Skip network discovery, interactive configuration, and pairing.
• Avoid Python virtual-environment recreation for native-only installations; repair the legacy Python environment only when required and unhealthy.
• Complete user-space preflight before requesting sudo.
• Replace only LG Buddy-owned runtime, integration, desktop, hook, and unit files.
• Reload and restart system and user services in a defined order.
• Verify the installed binary identity and report partial failures with recovery guidance.

Acceptance criteria

• Candidate-specific preflight is non-mutating and refuses incompatible layouts before sudo or installation writes.
• The upgrade path never invokes interactive configure.sh behavior or pairing.
• Configuration, update policy, and native credentials are byte-for-byte preserved.
• Native-only upgrades do not recreate the Python environment.
• Service/unit/hook replacement and post-install version verification are covered in the sandboxed bundle smoke test.
• Shell syntax and focused installer smoke coverage pass.

Out of scope

• Defining the complete or permanent host-compatibility check set; tracked and evolved in Add runtime host upgrade compatibility preflight #100.
• GitHub release discovery or downloading.
• Public lg-buddy updates install orchestration.
• Full transactional rollback after privileged mutation begins.

[Staphylococcus]

Implemented by PR #106 and released in v1.4.0. The release-bundle installer now supports the tested non-interactive upgrade path with state preservation. Release: https://github.com/Staphylococcus/LG_Buddy/releases/tag/v1.4.0