docs(dart): Draft New Spans and Migration Guide pages

[buenaflor]

DESCRIBE YOUR PR

Draft documentation for span streaming (stream mode) in the Dart and Flutter SDKs, mirroring the Python "New Spans" pages (#18456). Stream mode (SentryTraceLifecycle.stream) sends spans to Sentry as they finish instead of batching them into a transaction at the end.

• New Spans page (tracing/new-spans/): why/when to use stream mode, enabling it, manual instrumentation (startSpan / startSpanSync / startInactiveSpan, auto-parenting via zones, retroactive timing), typed SentryAttribute attributes, SentrySpanStatusV2, beforeSendSpan / ignoreSpans, sampling, and verification.
• Migration guide (tracing/new-spans/migration-guide/): maps the transaction-based APIs (startTransaction, startChild, setData/setTag, beforeSendTransaction) to the new span APIs.
• Pages live in common/ so both the Dart platform and the Flutter guide pick them up; init snippets are split via platform-includes (SentryFlutter.init vs Sentry.init).

API reference: getsentry/sentry-for-ai#164.

Open questions for SME review

• Confirm the minimum SDK version (>=9.19.0) is correct for traceLifecycle.
• The Python template has a distributed-tracing (continue/new trace) section; it was omitted here because the Dart stream-mode propagation APIs weren't in the reference. Add if they exist.

IS YOUR CHANGE URGENT?

Help us prioritize incoming PRs by letting us know when the change needs to go live.

• Urgent deadline (GA date, etc.):
• Other deadline:
• None: Not urgent, can wait up to 1 week+

SLA

• Teamwork makes the dream work, so please add a reviewer to your PRs.
• Please give the docs team up to 1 week to review your PR unless you've added an urgent due date to it.
  Thanks in advance for your help!

PRE-MERGE CHECKLIST

Make sure you've checked the following before merging your changes:

• Checked Vercel preview for correctness, including links
• PR was reviewed and approved by any necessary SMEs (subject matter experts)
• PR was reviewed and approved by a member of the Sentry docs team

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
sentry-docs	Ready	Preview, Comment	Jun 25, 2026 1:47pm

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
develop-docs	Ignored		Jun 25, 2026 1:47pm