text: remeasure list items when a document is replaced with an equal block count

[kossoy]

Document::render_root only calls ListState::reset when the block count
changes. The full-measure pass enabled by measure_all is a one-shot latch that
only reset, remeasure_items, or a width change re-arms. So replacing a
document with one that happens to have the same number of blocks leaves every
cached height belonging to the previous document.

The list summary height stays wrong, and since the wheel clamps against that
summary, the blocks past the false bottom can never scroll into view to be
re-measured. The clamp seals itself.

Reproduction (deterministic)

1. Open a document with 96 short blocks (e.g. 96 one-line paragraphs).
   max_off ≈ 3 024 px.
2. Replace it with a document that also has 96 blocks but is 132 KB long.
3. max_off is now ≈ 3 503 px instead of the true ≈ 21 314 px. Wheel-scrolling
   stops at 16 % of the document. Resizing the window "fixes" it, because a
   width change re-arms the latch — which is why this is hard to notice
   interactively.

Fix

Call remeasure_items(0..count) at the two places parsed_content is replaced
(sync and async full-parse paths). remeasure_items re-arms the latch and marks
items unmeasured while keeping the old sizes as hints, so the scrollbar does not
collapse for a frame.

Placement matters: hooking increment_update instead races on the async path —
a prepaint can land between the invalidation and the arrival of the new blocks
and re-latch against the old content. Both hooks sit at the exact instant
parsed_content is replaced; the async one is gated on full_parse so streaming
appends are unaffected.

Verified: the same repro after the fix ends at max_off = 21 314, the true
height, and a 500-tick wheel sweep reaches the final block. No gpui core change;
crates/base/src/text/state.rs only, +25 lines.