fix(core): reject non-JSON-safe layer keys with LayerKeyError

[SutuSebastian]

Summary

• Runtime-assert JSON-safe LayerKey segments in hashKey (assertLayerKey); throw LayerKeyError / narrow with isLayerKeyError
• Allowed: string | boolean | null | finite number | plain objects/arrays of those; reject undefined (incl. object props), NaN/±Infinity, bigint, symbol, functions, non-plain objects, cycles
• Docs: JSON-safe domain + prior [undefined]/[null]/[NaN] → "[null]" collision note; sync throw vs Promise reject in error-handling

Breaking note

Apps using undefined / NaN / non-finite numbers / BigInt / cycles / class instances in keys will start throwing. Fix: use null or explicit sentinels (filterId ?? "none"); omit optional object fields instead of setting undefined.

Test plan

• bun test packages/core/src/layerStack.test.ts (reject matrix + sync open throw)
• bun run --filter ./packages/core typecheck
• bun run docs:check --isolated + docs:build
• CI green

[changeset-bot]

🦋 Changeset detected

Latest commit: 4e45ecd

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 11 packages

Name	Type
@stainless-code/layers	Patch
@stainless-code/alpine-layers	Patch
@stainless-code/angular-layers	Patch
@stainless-code/layers-devtools	Patch
@stainless-code/lit-layers	Patch
@stainless-code/preact-layers	Patch
@stainless-code/react-layers-devtools	Patch
@stainless-code/react-layers	Patch
@stainless-code/solid-layers	Patch
@stainless-code/svelte-layers	Patch
@stainless-code/vue-layers	Patch

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Click here if you're a maintainer who wants to add another changeset to this PR

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Configuration used: defaults

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📥 Commits

Reviewing files that changed from the base of the PR and between 323be82 and 4e45ecd.

📒 Files selected for processing (14)

• .changeset/json-safe-layer-keys.md
• apps/docs/content/adapters/core.mdx
• apps/docs/content/concepts/glossary.mdx
• apps/docs/content/concepts/identity-and-types.mdx
• apps/docs/content/guides/error-handling.mdx
• apps/docs/content/reference/core-api.mdx
• docs/architecture.md
• docs/glossary.md
• packages/core/skills/layers/SKILL.md
• packages/core/src/errors.ts
• packages/core/src/index.ts
• packages/core/src/layerStack.test.ts
• packages/core/src/types.ts
• packages/core/src/utils.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/json-safe-layer-keys

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