Tier rail 3/4: rename showGlasses to "Show difficulty guide" and migrate the stored value #158

Part of #131 (remediation step 6). Can ship independently of the renderer.

showGlasses does not identify a stable location or granularity: with Section Map installed the standalone HUD is suppressed and relocated (via window.__slopsmithSectionMapHooksInstalled), so the setting means different things in different installs.

Scope

• Rename the setting to Show difficulty guide in settings.html (~L33-36) and the README settings table.
• Migrate the stored showGlasses value to the new key on first read (preserve an existing user's on/off choice; do not reset it to the default).
• State in the setting's help text, and in the README, where the guide appears when Section Map is installed.
• Do not read or write localStorage from a gameplay-event handler when doing the migration (cache the setting; see the repo CLAUDE.md).

Acceptance criteria

• A user with showGlasses off keeps the guide off after upgrading; one with it on keeps it on.
• The migration is idempotent and covered by a test (old key only, new key only, both present, neither).
• The README and plugin.json description no longer describe the setting in glass terms (the final glass-terminology sweep is sub-issue 4).

Out of scope

The renderer (sub-issue 2) and removing glassSize (sub-issue 4).