Memoize scanPrototypesFor to avoid per-instance prototype walks

[bemky]

Summary

• scanPrototypesFor(this.constructor, key) runs four times in KompElement's constructor (bindMethods, assignableAttributes, assignableMethods, events) and twice in TableColumn's — every cell, row, and column construction climbs the prototype chain and rebuilds the same arrays.
• The result depends purely on the (constructor, key) pair, so memoizing it gives every instance after the first a constant-time lookup.
• Cache is a WeakMap keyed by the constructor → Map keyed by property name → result array. The cache stores references to the per-prototype values (not copies), so plugin in-place mutations like this.events.push('columnResize', 'rowResize') in resizable.js remain visible whether they run before or after first scan.

Why this is safe

All current call sites use the pattern scanPrototypesFor(...).filter(x => x).reverse().forEach(...). filter returns a fresh array, and the subsequent reverse mutates that fresh array — the cached array is never touched. The doc comment notes that callers should treat the returned array as read-only.

Performance impact

Hottest path: building 1000×10 = 10,000 cells previously did 40,000 prototype walks (4 per cell). After this change it's 4 per Cell class total, then O(1) lookups thereafter.

Test plan

• Render a table/spreadsheet, confirm cells/rows/columns construct identically (attributes, events, methods all wired up).
• Confirm Spreadsheet.events includes plugin-added entries (columnResize, rowResize, etc.) and that onColumnResize constructor option still binds.
• Smoke-test docs build (npm run docs).

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