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refactor(virtual-list): remove the dead debounced measurement pipeline (#420)

Fixes #417

The #413 fix moved all measurement into a synchronous ResizeObserver
path; nothing has populated dirtyItems since, which left the entire
debounced pipeline unreachable:

- dirtyItems/dirtyItemsCount had no writers, so triggerHeightUpdate's
  guard never passed — the throttled body, its $effect, and the
  createAdvancedThrottledCallback machinery were dead
- updateHeight ran exactly once (the onMount post-hydration fallback)
  with heightChanges always empty; its only surviving effect was
  seeding the average estimate in environments where ResizeObserver
  never fires. Decision (a) from the issue: no non-RO fallback —
  ResizeObserver is the component's measurement mechanism and a
  universal baseline; without it the fallback never reached the
  manager's cache anyway (its callback discarded updatedHeightCache)
- the initialized-gated at-bottom correction living inside that
  callback dies with it — this resolves the at-bottom third of #416;
  end-stable-at-bottom behavior is owned by the anchor path's
  kind:'bottom' branch since #413

Deleted with their tests, per decision (b): heightCalculation.ts
(calculateAverageHeightDebounced's only caller was updateHeight),
calculateAverageHeight in virtualList.ts (only caller was
heightCalculation), and throttle.ts (only caller was
triggerHeightUpdate; not exported from the package). Component state
that existed solely for the pipeline (lastMeasuredIndex,
heightUpdateTimeout, isCalculatingHeight,
prevTotalHeightForScrollCorrection, HEIGHT_DEBOUNCE_MS) is gone, and
the stale architecture comments in the component header and CLAUDE.md
are updated.

Net -1,100 lines. Verified behavior-neutral: svelte-check clean, unit
tests 283 passed (the 43 removed tests covered only the deleted dead
utils), and the full Playwright suite identically green on chromium,
firefox, and webkit (204 passed / 0 failed — same counts as before the... (continued)

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