[codex] unify auto-start labels and clarify performance wording

[LiveTrack-X]

Summary

This change bundles two related cleanups that were already in the local worktree and reconciles them into a reviewable branch.

First, it unifies the auto-start label used by the tray menu and the Settings toggle so both surfaces read from a single platform-aware source. That removes the previous mix of Windows-specific and per-file hardcoded wording and keeps the UI consistent across platforms.

Second, it tones down several documentation statements that could over-promise behavior around scanning, preset switching, monitor latency, receiver latency, and long-duration stability. The updated wording keeps the product strengths visible, but narrows claims to what the current implementation actually guarantees under typical conditions.

User impact

Users should see a more consistent auto-start label in the app UI, and the public docs should now set clearer expectations about performance-sensitive behavior. This reduces the chance of confusion around what is instantaneous, what is usually fast, and what remains hardware- or plugin-dependent.

Root cause

The auto-start wording had diverged because multiple UI surfaces defined labels locally instead of sharing one source of truth. Separately, documentation had accumulated several stronger marketing-style phrases than the code strictly guarantees, especially in areas where behavior depends on plugin composition, cache state, driver quality, or host configuration.

Fix

• Added Platform::getAutoStartLabel() and used it from both the tray menu and Settings toggle.
• Updated the changelog to note the UI-label consistency change.
• Reworded README and architecture docs to replace absolute or unconditional phrasing with bounded, implementation-aligned wording.

Verification

• Fetched all remotes and confirmed no new upstream commits needed reconciliation before branching.
• Built the host app target successfully with MSBuild (DirectPipe.vcxproj, Debug).
• Built the host test target successfully with MSBuild (directpipe-host-tests.vcxproj, Debug).
• Ran directpipe-host-tests.exe: 244 tests executed, 242 passed, 2 skipped.