(v1) Forward-port: optional shared.ts for Nuxt strict auto-extend

[yamcodes]

Problem

On v1, #1422 made Nuxt strict layout auto-extend SharedSchema via #arkenv/shared-schema and hard-fails when env/internal/shared.ts is missing (missingSharedTsError). That is louder than the pre-existing @arkenv/build shared-file gate and blocks a client + server–only strict setup even after the layout layer no longer requires the file.

The base layout / docs / CLI change lands on dev in #1448. This issue is the v1 forward-port plus the Nuxt-specific soft-fail that only exists on v1.

Related: #1448 (base on dev), #1414, #1422

Desired behavior

• Forward-port Make env/internal/shared.ts optional in strict layout (dev/v0) #1448’s @arkenv/build layout detection / findSchemaPath / docs / CLI wording onto v1.
• When internal/shared.ts is absent in Nuxt strict layout, #arkenv/shared-schema / client auto-extend uses an empty schema (same fallback already used outside strict via empty-shared-schema.ts), instead of missingSharedTsError.
• When the file exists but has no usable SharedSchema export, fail with a clear diagnostic (broken file ≠ intentional empty).
• Present SharedSchema still auto-merges when extends is omitted.

Acceptance criteria

• Make env/internal/shared.ts optional in strict layout (dev/v0) #1448 is merged on dev and this work is unblocked.
• resolveLayout / findSchemaPath on v1 match Make env/internal/shared.ts optional in strict layout (dev/v0) #1448 (shared file not required for strict).
• Nuxt strict client/server auto-extend works with no shared file (empty merge).
• Present SharedSchema still auto-merges when extends is omitted.
• A present shared file without a usable SharedSchema export still fails with an actionable diagnostic.
• Docs/CLI on v1 describe shared as optional; CLI still scaffolds shared.ts by default.

Out of scope

• Re-doing the base layout design (owned by Make env/internal/shared.ts optional in strict layout (dev/v0) #1448)
• Making client.ts or server.ts optional
• Configurable sharedPath
• Changing Next.js manual extends: [SharedSchema] scaffolding beyond the shared layout-detection forward-port

Target branch

v1 (blocked on #1448).

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[yamcodes]

Forward-port of #1448 onto v1, plus the Nuxt auto-extend / #arkenv/shared-schema empty-schema behavior introduced by #1422 (replace hard-fail when internal/shared.ts is absent).

Blocked until #1448 lands on dev.