docs(start): document serialization adapters

[aakbarpour]

Summary

• update the Router SSR serialization guidance
• document custom serialization adapters in Start
• clarify built-in serialization and FormData handling

Closes #7796

Testing

• corepack pnpm test:docs
• Prettier check
• affected ESLint, type, and unit checks

Summary by CodeRabbit

• Documentation
  • Expanded SSR serialization guidance for JSON-compatible values, special types, nested and cyclic references, and FormData.
  • Added guidance for custom serialization adapters in TanStack Start.
  • Included an example showing how to serialize and deserialize application-specific values such as currency amounts.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 4b8f80c4-666e-4793-98eb-ee9b2295f502

📥 Commits

Reviewing files that changed from the base of the PR and between 9f8990b and 9c7bbe5.

📒 Files selected for processing (2)

• docs/router/guide/ssr.md
• docs/start/framework/react/guide/server-functions.md

Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review.

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📝 Walkthrough

Walkthrough

The documentation updates describe SSR serialization support and custom serialization adapters in TanStack Start. They cover supported value types, reference preservation, adapter registration, adapter hooks, and adapter configuration fields.

Changes

Serialization Documentation

Layer / File(s)	Summary
SSR serialization support docs/router/guide/ssr.md	The SSR guide lists supported JSON-compatible values and special types, including undefined, BigInt, Date, Error, Map, and Set. It also documents nested and cyclic references, FormData scope, and custom serialization adapters.
Start adapter registration and usage docs/start/framework/react/guide/server-functions.md	The guide documents custom adapter use during hydration and server-function serialization. It adds a Money adapter example, createStart registration, and descriptions of the adapter configuration fields.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 9c7bb

This PR only updates serialization documentation and introduces no production behavior changes; no actionable merge-blocking risk remains after normal checks and review.

Suggested reviewers: tannerlinsley, schiller-manuel

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the main change: documenting serialization adapters for TanStack Start.
Linked Issues check	✅ Passed	The documentation updates address the linked issue by covering adapters, registration, SSR and server functions, custom types, and built-in BigInt support [#7796].
Out of Scope Changes check	✅ Passed	All changes update related Router SSR and TanStack Start serialization documentation and remain within the linked issue scope.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (2 skipped: 2 unsupported.)

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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