docs: document sash.store one-way-seed intent and tech debt

[ohxyz]

What

Expands the comment on Sash.store (src/sash.js) from a one-liner into a /* */ block documenting its intended design and a known tech-debt gap.

Why

store is meant to be a one-way construction seed — config props flow from the top-level API into pane/glass construction, then stop. Data exchange for moves (attach/detach/swap) happens at the DOM layer, which is visible and debuggable in the browser.

But the code doesn't fully honor that yet: onDrop (droppable.js) is read at drop time and resizable (muntin.js) when a pane is split — both long after construction. So anyone who tries to clear store in onPaneCreate (a reasonable thing to attempt) would silently break per-pane onDrop callbacks and resizable-on-split.

The comment now records:

• RATIONAL — the one-way-seed / DOM-is-the-exchange-layer intent.
• TECH DEBT — the two late readers, and the concrete consequence: store can't be cleared until onDrop/resizable are moved onto the DOM/closures at build time.

Scope

Comment only — no behavior change.