[Interactive Graph] Add WB Announcer to Exponential graph

[catandthemachines]

Summary

The Exponential graph has two distinct keyboard moves, and this wires both into the WB Announcer:

• Control points (movePoint indices 0/1, MovablePoint) — the reducer's doMovePointInFigure case "exponential" now emits a move-exponential-point stateAnnouncement. getAnnouncementText reads index 0 as "Point 1 at X comma Y." and index 1 as "Point 2 at X comma Y." (via a new srExponentialPointLabel helper), with an author-supplied custom label ("Point A at X comma Y.") overriding when one is set — mirroring sinusoid's pointLabels handling.
• Horizontal asymptote (moveCenter, MovableAsymptote) — doMoveCenter's case "exponential" now emits a move-exponential-asymptote stateAnnouncement carrying the new y, read via the existing srExponentialAsymptote string.
• exponential.tsx passes ariaLive="off" to both the MovablePoints and the MovableAsymptote so moves aren't double-announced. MovableAsymptote gains an optional ariaLive prop that defaults to "polite", leaving any not-yet-migrated asymptote graph that shares the component unchanged.
• Both reducer paths keep their existing rejection checks first, so a rejected move emits no stateAnnouncement.
• TODOs reference LEMS-4189, which tracks removing aria-live from useControlPoint / MovableAsymptote once every graph is wired to the announcer.

Note

The asymptote announcement reuses srExponentialAsymptote ("Horizontal asymptote at y equals Y. Use up and down arrow keys to move."), preserving today's behavior exactly. The keyboard hint now repeats on every keypress through the announcer — a position-only string could be a nice follow-up.

The MovableAsymptote ariaLive prop is also introduced by the Logarithm PR (LEMS-4194). If both merge, whichever lands second needs a trivial rebase on that shared component change.

Issue: LEMS-4193

Test plan

Automated

• pnpm tsc — passes
• pnpm lint (changed files) — passes
• pnpm jest packages/perseus/src/widgets/interactive-graphs — passes (reducer, screenreader-text, exponential, and movable-asymptote suites)

Manual (reviewer)

• Open the Interactive Graph → Exponential story in Storybook with VoiceOver/NVDA (or the a11y addon's live-region inspector).
• Arrow-move point 1 and point 2: confirm you hear "Point 1/2 at X comma Y." once each — not doubled.
• Tab to the horizontal asymptote and arrow-move it: confirm you hear "Horizontal asymptote at y equals Y…" once.
• With custom point labels set, confirm a labeled point announces "Point A at …" on move.
• Regression: open the Logarithm story and confirm its asymptote/point announcements still work as expected.

[github-actions]

npm Snapshot: Published

Good news!! We've packaged up the latest commit from this PR (6afe1f8) and published it to npm. You
can install it using the tag PR3736.

Example:

pnpm add @khanacademy/perseus@PR3736

If you are working in Khan Academy's frontend, you can run the below command.

./dev/tools/bump_perseus_version.ts -t PR3736

If you are working in Khan Academy's webapp, you can run the below command.

./dev/tools/bump_perseus_version.js -t PR3736

[github-actions]

Size Change: +74 B (+0.01%)

Total Size: 509 kB

📦 View Changed

Filename	Size	Change
packages/perseus/dist/es/index.js	200 kB	+74 B (+0.04%)

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	6.32 kB
packages/math-input/dist/es/index.js	98.5 kB
packages/math-input/dist/es/strings.js	1.61 kB
packages/perseus-core/dist/es/index.item-splitting.js	12.1 kB
packages/perseus-core/dist/es/index.js	26.3 kB
packages/perseus-editor/dist/es/index.js	105 kB
packages/perseus-linter/dist/es/index.js	9.8 kB
packages/perseus-score/dist/es/index.js	10.2 kB
packages/perseus-utils/dist/es/index.js	403 B
packages/perseus/dist/es/strings.js	8.6 kB
packages/pure-markdown/dist/es/index.js	1.39 kB
packages/simple-markdown/dist/es/index.js	6.71 kB

_{compressed-size-action}