fix(surveys): exclude web-only display conditions on non-web SDKs

[posthog]

Why

posthog-ios#733 ("don't show web-only surveys (selector/url) on iOS") fixed iOS's getActiveMatchingSurveys eligibility filter, which decoded a survey's conditions.selector/conditions.url display-targeting fields but never evaluated them — a survey scoped to web via CSS selector/URL matching rendered natively on iOS with non-functional buttons, since those conditions have no meaning outside a browser DOM. The PR description claims this "keeps parity with the other native SDKs (Android, Flutter, React Native)."

That claim only holds up partially. I checked source directly rather than trusting the PR description:

• React Native (posthog-js's getActiveMatchingSurveys.ts) already excludes selector/URL-only-conditioned surveys, with dedicated test coverage — it was never affected.
• Android (PostHogSurveysIntegration.getActiveMatchingSurveys()) never reads conditions.selector/conditions.url at all — the exact same gap iOS just fixed, still present.
• Flutter has no independent eligibility logic; it delegates to whichever native SDK it wraps, so it inherits Android's gap on Android and iOS's (now-fixed) behavior on iOS.

openspec/specs/surveys/spec.md behavior item 5 only names "platform-specific display constraints" — vague enough to not actually state this rule. Per this repo's stated purpose (state the canonical winner where SDKs diverge), React Native's pre-existing correct behavior — now matched by iOS — is the target; Android and Flutter are documented as not yet conforming rather than silently assumed fixed.

What this PR does

• Names the concrete exclusion rule in Behavior item 5 instead of the vague "platform-specific display constraints" phrase.
• Adds one new @client scenario to the Canonical surveys behavior requirement (via openspec/changes/archive/2026-07-31-exclude-web-only-survey-conditions/, following this repo's propose→archive convention).
• openspec validate --specs --strict passes.

Uncertain / flagged for reviewer attention

• posthog-android does not conform — confirmed by reading PostHogSurveysIntegration.kt directly, not just trusting the iOS PR's parity claim. This PR only documents the target contract; it does not fix Android or open tracking work for it (noted in tasks.md §4).
• posthog-flutter's conformance is inherited from whichever native SDK it wraps at runtime, so it's non-conforming on Android today too.

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