docs: plan Linux support implementation

[enaboapps]

Linux is represented in capabilities but lacks a native runtime and cursor-overlay backend. This adds an ordered implementation plan covering BlueZ peripheral interoperability, secure sessions, X11 input, desktop integration, packaging and updates, and a separate Wayland qualification milestone.

The plan defines support boundaries, source references, acceptance criteria, hardware and regression testing, and unresolved backend decisions. This PR delivers documentation only; implementation will use follow-up issues.

Closes #710.

Validation: git diff --check passed and all 19 local documentation links resolve. Independent source review of head ee8074b4370f113455503d525509bb20a22d92f6 found no actionable substantive issues. All CI checks passed: frontend, dependency audit, Windows/macOS native tests and packaging, and CodeQL. Greptile review also passed. No Linux runtime or hardware validation was performed for this documentation-only change.

[greptile-apps]

Greptile Summary

This PR adds a proposed, phased plan for delivering Linux support while keeping the initial release boundary explicit: Ubuntu 24.04 x86_64 with an X11 session. It preserves existing macOS and Windows behavior as a requirement, defers native Wayland support to separate compositor-specific qualification, and defines security, transport, input, lifecycle, packaging, and release evidence needed before Linux is enabled. The documentation check confirmed that all 19 repository-relative links resolve, the declared d222639 baseline corresponds to v1.0.0-rc.6, and the plan is the only changed path.

Confidence Score: 5/5

Safe to merge: this change adds planning documentation only and does not alter shipped application behavior.

The documentation contract check passed: all repository-relative references resolve at the current revision, and the stated baseline, version, and documentation-only scope match the repository state.

Files Needing Attention: No files need corrective changes. Future implementation work should use docs/linux-support-plan.md as the release and validation checklist.

T-Rex Logs

What T-Rex did

• We captured the declared baseline with git show d222639:docs/linux-support-plan.md and confirmed the file was absent at that baseline, with docs/linux-support-plan.md identified as the sole changed path through HEAD.
• We ran the linux-support-plan contract checker at HEAD and it exited successfully with repository_relative_links_checked=19, changed_paths_since_baseline=docs/linux-support-plan.md, and RESULT=PASS.
• The check validated that the linux-support-plan is additive documentation, its repository-relative links resolve, and the declared baseline and version match the repository state.
• Before capture, git show d222639:docs/linux-support-plan.md exited 128 because the plan was absent at the declared source baseline; after capture, the checker exited 0 with the same metrics and the result remains PASS, while no security issue is proven.
• Artifacts were collected to support review, including the contract checker source and the baseline and HEAD check outputs.

_{Ran code and verified through T-Rex}

_{Reviews (1): Last reviewed commit: "docs: plan Linux support implementation ..." | Re-trigger Greptile}