feat: report reason for recording start

[pauldambra]

in this private slack thread https://posthog.slack.com/archives/C06R7TKD508/p1727988694238459

A user has

• 10% sampling set on replay
• a batch export of events that identify interesting sessions
• every session has a session id regardless of it was recorded
• so the user has to check every session id one at a time to see which was recorded

adding this as an event has cost implication for users (if small)
and it is tricky to pick up a definitive start reasons for some flows as start is idempotent and we call it more than once

so, let's add a session super property so folk can look for that

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Name	Status	Preview	Updated (UTC)
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[github-actions]

Size Change: +5.23 kB (+0.19%)

Total Size: 2.83 MB

Filename	Size	Change
dist/array.full.js	354 kB	+581 B (+0.16%)
dist/array.full.no-external.js	353 kB	+581 B (+0.16%)
dist/array.js	169 kB	+581 B (+0.35%)
dist/array.no-external.js	168 kB	+581 B (+0.35%)
dist/main.js	170 kB	+581 B (+0.34%)
dist/module.full.js	354 kB	+581 B (+0.16%)
dist/module.full.no-external.js	353 kB	+581 B (+0.16%)
dist/module.js	169 kB	+581 B (+0.35%)
dist/module.no-external.js	168 kB	+581 B (+0.35%)

ℹ️ View Unchanged

Filename	Size
dist/all-external-dependencies.js	192 kB
dist/exception-autocapture.js	11.1 kB
dist/external-scripts-loader.js	2.35 kB
dist/recorder-v2.js	111 kB
dist/recorder.js	111 kB
dist/surveys-preview.js	59.8 kB
dist/surveys.js	66 kB
dist/tracing-headers.js	8.36 kB
dist/web-vitals.js	10.3 kB

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[pauldambra]

i'm going to make this draft... turns out to be really tricky to only trigger once on "normal" start since we might need to wait for decide or not

also makes me think it'd be useful in a recording/session to be able to distinguish between client and browser navigation generally

[posthog-bot]

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