fix: blank glass on restore-minimized via addPane withGlass flag

[ohxyz]

Closes #61

What

BinaryWindow.addPane always seeded a placeholder Glass built from its glassProps, even for callers that already hold a real glass element and only need the pane. This conflated two intents in one signature and forced those callers to defeat the placeholder.

The restore-minimized path (glass/action.js) used append (not replaceChildren), leaving two bw-glass children in the pane — empty placeholder first, restored glass second. A later detach reads querySelector('bw-glass-content'), which returns the first (empty) match → a blank glass (obvious when maximized). This is the same class of bug as #56 (edge-split drop).

Change

• Add a withGlass flag to addPane (defaults to true). When false, addPane allocates a bare pane without constructing a glass.
• glass/drag.js and glass/action.js (the bring-your-own-glass callers) pass withGlass: false and append their own glass into the now-empty pane.
• Build-a-glass callers (detached-glass/action.attach.js, the dev/ add-pane page) are unchanged — withGlass defaults true, so the seeded glass and the react-bwin addPane content workaround keep working.
• Removed the corresponding docs/TECH_DEBT.md entry (per its "delete when fixed" convention).

Notes

• drag.js previously used replaceChildren to overwrite the placeholder; with no placeholder, a plain append into the fresh empty pane is equivalent and keeps fix: detached glass empty after edge-split drop #56 fixed.
• Not yet verified live — recommend exercising drag-out → edge-drop → maximize and restore-minimized → detach.