docs(mta): multi-touch attribution guide (LIN-2958)

[tashviks]

Customer guide for the Multi-touch page (features/multi-touch-attribution.mdx, linked in docs.json) for LIN-2958, with the view journey rules from LIN-3365.
It covers the page as it ships: Overview and Journeys, how credit is shared, which touches count, and when the page shows no numbers.

Commits

• 1f00403 first version of the guide.
• bf019df view journeys: affiliate impressions and Meta views count as touches where view journeys are switched on (hazelnut MTA_VIEWS_ON_PATHS, default off); a click outranks a view within one network; Google views never enter a journey.
• 5848029 rewrite for the current page: Overview and Journeys only (the hidden Campaigns, Ad sets and creatives and Google view-through tabs, and the lift test and recommendation sections, are gone); plain names only (install impact, a statistical model; no algorithm names); last touch for the touch that won the install, which the page labels Last click; the eligibility messages the page shows.
• b0d69b3 drops the From views column from the Overview table (views are inside the totals) and names the journey funnel columns Second assist, First assist and Last touch, as the page now heads them.
• f531aff adds the Third assist column to the journey funnel and the 4 or more bucket to Journey length.
• 6649b71 merge of main (62 commits behind; no conflicts).

Review notes

• Sources: the dashboard's metrics guide (linkrunner-dashboard #783 docs/multi-touch-attribution-metrics.md) and the hazelnut job (#859). The page is not live yet, so claims are checked against those branches, not production.
• The page says Last click in the table headers and roles; the guide says last touch and names the label once. If the page is renamed to Last touch, the guide only needs that one sentence changed.
• Checks: npm run check:agents 9/9 pass; npm run check:routes -- origin/main valid, 127 pages. Not run: mintlify dev render.
• Opened as a draft (my working rule), not the non-draft PR with RathodDarshil as reviewer that this repo's AGENTS.md asks for; switch it when ready.

Screenshots

Five screenshots from the live demo dashboard (demo.linkrunner.io, synthetic data) in images/multi-touch-attribution/: Overview tiles, the charts row, the channel and campaign tables, and the Journeys funnel. The tile and column tables now match the page (Top assister and Top last click tiles, Won instead of Vs last click, the pinned Organic row, the Campaigns switch label). The demo lags the dashboard in two places: Journey length shows "3 or more steps" (dashboard: "4 or more") and the funnel has no column headings (dashboard: Third assist to Last touch).

Counterpart PRs

• hazelnut: https://github.com/linkrunner-labs/hazelnut/pull/859
• backend: https://github.com/linkrunner-labs/linkrunner-backend/pull/1277
• dashboard: https://github.com/linkrunner-labs/linkrunner-dashboard/pull/783
• ops: https://github.com/linkrunner-labs/ops/pull/1028

Merging publishes the docs site, so it waits for the page to ship.

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