Capitalize State in docs

[Dextheking1]

Part of #6713 (State item).

Capitalizes the React concept State where prose refers to the concept directly (per the maintainer's guidance in #6717: capitalize only where it "makes sense to link to the reference page"). 8 casing-only edits across 5 files:

• learn/adding-interactivity.md: term definition ("is called State"); "(State is one of those features)" mirrors the existing "State is just one of those features"
• learn/state-a-components-memory.md: term definition; "State is fully private to the component declaring it" (subject-as-concept)
• learn/state-as-a-snapshot.md: "State is not like a regular variable" (fixes intra-paragraph inconsistency -- the next sentence already reads "State actually lives in React itself")
• reference/react/useState.md: "In React, State is considered read-only" (mirrors existing "State is read-only" in useReducer.md); link text "[State behaves like a snapshot.]" (also fixes subject-verb agreement)
• reference/react/useReducer.md: link text "[State behaves like a snapshot.]"

Deliberately left lowercase (reviewed ~1,380 prose occurrences; no blind replacement):

• descriptive/generic uses: "the component's state", "update/set/reset state", "a piece of state", "local/global/shared state", "state variable/updater/logic", "loading states", "visual states"
• "React state" as an established descriptive compound (the docs contain zero instances of "React State")
• headings, URLs, anchors, inline code, and identifiers untouched

Excludes blog/, community/, and the homepage, per the maintainer's feedback on #6717.

[github-actions]

Size changes

Details

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

This PR introduced no changes to the JavaScript bundle! 🙌