Pairing survives a project whose rules are older than the app

[jsconu]

Fixes the "That code isn't active ... the code was found, but linking this phone was refused" failure that made pairing impossible on a Firebase project whose firestore.rules hadn't been re-published since #39.

What was wrong

Claiming a code wrote three documents in one transaction: the child profile, the code doc, and the kid device's linkedDevices doc. That third write only became legal in the rules in the same commit that added it — so if the deployed rules were older than the app, Firestore refused the entire transaction. The code doc itself stayed readable, which is why the error said the code was found.

Reproduced in the Firestore emulator

Running the repo's real rules, with and without the linkedDevices block:

	rules without linkedDevices	current rules
claim with the link write inside the transaction (before)	permission-denied	pairs
claim with the link write after it (after)	pairs, link refused and ignored	pairs, links

Changes

• PairingRepository — the transaction holds only the two writes that are the pairing. The linkedDevices doc is written straight after, best effort. The Residual security and reliability items from the #38 review #39 security binding is unchanged: getAfter still proves this device claimed that parent's code.
• firestore.rules — resource.data.get('deviceUid', null), because reading a field that isn't there is an evaluation error, not null, and denied the claim outright.
• ChildProfile.swift — assigning nil to a Swift dictionary subscript deletes the key, so every child created in the iOS parent app was written without deviceUid and could never be paired. Writes NSNull() now, matching the rest of that file.
• The claim's error path no longer marks the code used while diagnosing which step was refused.
• Regression test for a child doc with no deviceUid field, and a README note to re-publish the rules whenever that file changes.

Verification

:shared:testDebugUnitTest and :shared:compileDebugAndroidTestKotlin pass locally; the emulator matrix above is the substantive check. Confirmed working on a real phone against the live project after re-publishing the rules.

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