contrib: add migration checklist for breaking session schema changes

[arandomogg]

Summary

Adds a self-contained reference under contrib/examples/issue-282-session-schema-migration-checklist/ providing a standard checklist for safely rolling out a breaking change to the stored session schema across consumer versions.

Scoped entirely to contrib/ per CONTRIBUTING.md and contrib/README.md.

Why it's needed

WalletSession is persisted by the consumer's app (localStorage, or their own SessionStorageAdapter), not by the SDK's own process. A user can carry a session written by an older SDK for months before their app upgrades. restore() in src/session.ts is deliberately fail-soft — unreadable storage means "disconnected", never a crash — so an unmigrated breaking schema change silently signs every existing user out on their next app load, all at once, with no error surfaced anywhere.

Files

File	What it is
CHECKLIST.md	The checklist — backward compatibility, migration helper usage, testing, rollout — plus the worked example.
session-schema-migration-checklist.ts	A runnable implementation of that worked example, so the guidance is demonstrably correct rather than only asserted.
session-schema-migration-checklist.test.ts	18 tests exercising every item in checklist sections 1–3.
README.md	Orientation and how to reference the checklist from a migration PR.

The checklist covers

1. Backward compatibility — old sessions still parse, defined fallbacks for newly required fields, the guard is widened rather than disabled.
2. Migration helper usage — migrate on read (not write), pure and synchronous (no network or passkey prompt), idempotent, non-mutating.
3. Testing — old-shape input, new-shape passthrough, invalid-data rejection, and the fallback value.
4. Rollout — changelog entry under a Breaking heading, a stated compatibility window, and a tracked follow-up to drop the old read path.

Worked example

Replacing the flat lastActiveAt: string with a structured activity: { lastActiveAt, lastActiveNetwork }. The guard accepts both shapes, a pure and idempotent migrateSession upgrades old data on read with a defined fallback (the session's own network) for the field that didn't exist before, and invalid data still returns null.

Requirements checklist

• Checklist covering backward compatibility and migration helper usage
• Required to be referenced in migration PR descriptions (stated in CHECKLIST.md and README.md)
• Example walkthrough using a hypothetical schema change — and it's runnable and tested, not just prose
• Stored alongside the contribution guidelines (in contrib/, since this PR is scoped there)

Test plan

npx vitest run contrib/examples/issue-282-session-schema-migration-checklist

18 tests, all passing.

closes #282

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