fix(identify): document anonymous-to-identified transition on matching distinct id

[posthog]

Why

Three SDKs independently shipped the identical bug fix within days of each other:

• posthog-js#4328 — "identify anonymous users with matching IDs"
• posthog-ios#742 — "identify anonymous user when id already matches persisted distinct id"
• posthog-android#666 — same title, explicitly ported from iOS

The bug: calling identify(id) with an id that already equals the persisted distinct id, while the SDK still considers the user anonymous (e.g. a bootstrap-seeded id), fell into the "same distinct id" branch, which only acts when properties are supplied. With none, the call was dropped entirely — no event, and the identified-state flag never flipped to true. The person profile stayed anonymous indefinitely despite an explicit identify call.

The fix (identical across all three): treat this as its own identity-state transition — mark the user identified and emit exactly one person-processed $set (not $identify, since there's no anonymous id to merge), carrying any supplied properties (or empty $set/$set_once if none). This fires even with no properties supplied — the transition itself is the effect being recorded. Feature flags reload only if properties were supplied.

openspec/specs/identify/spec.md line 112 documented the old, buggy behavior as canonical ("Same distinct id, no properties → log 'already identified', drop"), with no carve-out for the anonymous case — this isn't a gap, the spec names the wrong winner.

What this PR does

• Adds one new @client scenario to the Canonical identify behavior requirement (via openspec/changes/archive/2026-07-31-fix-identify-anonymous-transition/, following this repo's propose→archive convention).
• Splits the old "Same distinct id" Behavior-section bullet into anonymous-transition vs. already-identified cases, and updates the "Duplicate-call suppression" summary table row to match.
• openspec validate --specs --strict passes.

Uncertain / flagged for reviewer attention

• Only posthog-js, posthog-ios, and posthog-android were checked. Flutter and React Native weren't audited for the same bug — worth a follow-up pass (noted in tasks.md §4).
• Server SDKs are unaffected (no anonymous-id concept per this spec's existing client/server split).

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