[Interactive Graph] Add WB Announcer to Tangent Graph

[catandthemachines]

Summary

• Wires the Tangent graph's inflection point (movePoint index 0) and second/control point (movePoint index 1) keyboard moves into the WB Announcer — the reducer's case "tangent" now emits a move-tangent-point stateAnnouncement, and tangent.tsx passes ariaLive="off" to MovablePoint so moves aren't double-announced.
• getAnnouncementText handles move-tangent-point via a new srTangentPointLabel helper: index 0 reads as the inflection point ("Inflection point at X comma Y.") and index 1 as the control point ("Control point at X comma Y."), matching the static aria-labels. An author-supplied custom label ("Point A at X comma Y.") overrides the inflection/control-point phrasing when one is set — mirroring sinusoid's pointLabels handling.
• The same-x rejection (which keeps the tangent coefficients defined) happens before the announcement, so a rejected move emits no stateAnnouncement.
• TODOs reference LEMS-4189, which tracks removing aria-live from useControlPoint once every graph is wired to the announcer.

Issue: LEMS-4196

Test plan

Automated

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

Manual (reviewer)

• Open the Interactive Graph → Tangent story in Storybook with VoiceOver/NVDA (or the a11y addon's live-region inspector).
• Arrow-move the inflection point: confirm you hear "Inflection point at X comma Y." once — not doubled.
• Arrow-move the second point: confirm you hear "Control point at X comma Y." once.
• With custom point labels set, confirm a labeled point announces "Point A at …" on move.
• Regression: open Sinusoid, Absolute Value, and Linear and confirm their announcements are unchanged.

[github-actions]

npm Snapshot: Published

Good news!! We've packaged up the latest commit from this PR (7295b9a) and published it to npm. You
can install it using the tag PR3734.

Example:

pnpm add @khanacademy/perseus@PR3734

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

./dev/tools/bump_perseus_version.ts -t PR3734

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

./dev/tools/bump_perseus_version.js -t PR3734

[github-actions]

Size Change: +60 B (+0.01%)

Total Size: 508 kB

📦 View Changed

Filename	Size	Change
packages/perseus/dist/es/index.js	200 kB	+60 B (+0.03%)

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	9.65 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

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