Fix migrate() bug and clarify ExtendRef requirements in documentation

[Copilot]

Addresses review feedback from PR #9 identifying a bug in migrate() functions and unclear documentation about ExtendRef requirements.

Bug Fix

• Move object_signer call inside existence check: Previously called before checking if Capability<OldProtocol, R> exists, causing abort even when migrating capabilities that don't exist, breaking the intended silent no-op behavior.

// Before: aborts if R doesn't exist but ExtendRef is missing
let object_signer = object_signer(old_protocol, object_address);
if (exists<Capability<OldProtocol, R>>(object_address)) { ... }

// After: only calls object_signer when R actually exists
if (exists<Capability<OldProtocol, R>>(object_address)) {
    let object_signer = object_signer(old_protocol, object_address);
    ...
}

Documentation Improvements

• migrate() functions: Clarify that ExtendRef under OldProtocol is required for all migrations, not just when migrating ExtendRef itself
• migrate_all() functions: Document that capabilities must be in storage (not extracted) and missing capabilities are silently skipped
• Inline comment: Expand explanation of why ExtendRef must be migrated last in migrate_all()

Applied to both protocol_object and protocol_fungible_asset modules.

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