[CLASS-13991] Stop swapping scroll caret icons on isRTL

[maddy531]

Summary

PhosphorIcon mirrors directional glyphs automatically in RTL once the
wonder-blocks-icon major lands (Khan/wonder-blocks#3174). This adapts perseus
to that, in the two places it matters:

scrollable-view.tsx — picking the caret from isRTL double-flips against
auto-mirroring, so it now passes the LTR-facing carets and lets mirroring handle
direction. That leaves isRTL with no readers on the file-local ScrollState
interface, so it's dropped; newIsRtl still drives the
canScrollStart/canScrollEnd math and stays.

locked-polygon-settings.tsx — the "move polygon left/right" arrows point
along the graph's x-axis, which is absolute: "left" means decreasing x in every
locale. Auto-mirroring would leave the button pointing right while still moving
the polygon left, so they opt out with mirrorInRtl={false}. IconButton
doesn't forward that prop, so they pass a PhosphorIcon element as icon
instead — IconButton accepts PhosphorIconAsset | React.ReactElement. The
up/down arrows need nothing, since only horizontal glyphs are mirrored.

Left alone: text-align-left/right in debug-header.tsx — physical
alignment, deliberately excluded from the mirroring allowlist.

Ordering

⚠️ Blocked on Khan/wonder-blocks#3174 publishing. Two reasons:

1. Typecheck fails until then. mirrorInRtl doesn't exist in
   wonder-blocks-icon 5.3.23, which perseus is pinned to. The red check is
   expected, not a real break.
2. The WB bump lands in this PR. Once the major publishes, bump both catalog
   entries in pnpm-workspace.yaml (^5.3.23 peer, 5.3.23 dev) in the same
   commit. That turns the typecheck green and makes the change coherent.

Perseus declares wonder-blocks-icon as a peerDependency, so it runs against
frontend's WB version. After this publishes, frontend bumps
@khanacademy/perseus and @khanacademy/wonder-blocks-icon together in one
PR (Khan/frontend#15137) — both are exact-pinned in the same catalog there, so
they land atomically with no window where these carets render against an
un-bumped WB.

A standalone perseus bump reaching frontend before that combined PR would show
two wrong-facing scroll arrows in RTL for one cycle. Worth avoiding.

Issue: CLASS-13991

Test plan

• pnpm jest scrollable-view — 10/10, including the three RTL cases. They
  assert on aria-label (scrollStart/scrollEnd) rather than glyph, so
  they cover behavior and stay valid.
• pnpm tsc — currently fails on mirrorInRtl (see Ordering). Pre-existing
  @dnd-kit/react resolution errors in dnd-kit-demo.stories.tsx are
  unrelated.
• Visual check in RTL once the WB bump is in: scroll carets point into the
  content, and the graph editor's left/right arrows still match the
  direction the polygon moves.

[github-actions]

npm Snapshot

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[github-actions]

Size Change: +10 B (0%)

Total Size: 519 kB

📦 View Changed

Filename	Size	Change
packages/perseus-editor/dist/es/index.js	106 kB	+32 B (+0.03%)
packages/perseus/dist/es/index.js	203 kB	-22 B (-0.01%)

ℹ️ View Unchanged

Filename	Size
packages/kas/dist/es/index.js	20.6 kB
packages/keypad-context/dist/es/index.js	1 kB
packages/kmath/dist/es/index.js	6.31 kB
packages/math-input/dist/es/index.js	98.6 kB
packages/math-input/dist/es/strings.js	1.63 kB
packages/perseus-core/dist/es/index.item-splitting.js	13.1 kB
packages/perseus-core/dist/es/index.js	28.3 kB
packages/perseus-linter/dist/es/index.js	10.1 kB
packages/perseus-score/dist/es/index.js	9.86 kB
packages/perseus-utils/dist/es/index.js	403 B
packages/perseus/dist/es/strings.js	12.9 kB
packages/pure-markdown/dist/es/index.js	1.39 kB
packages/simple-markdown/dist/es/index.js	6.12 kB

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