feat: vararg overloads for the write set actions

[zantvoort]

Summary

• All seven write set actions accept entities as varargs next to the canonical Iterable forms: insert, insertAndFetch, update, updateAndFetch, upsert, upsertAndFetch and remove. The flagship call shape reads as intended: orm.writeSet().insert(wolfie, rex, visit).
• The overloads are default methods delegating to the Iterable forms, so WriteSetImpl is untouched. Entity<?> is reifiable, which keeps the varargs warning-free without @SafeVarargs (which default interface methods cannot carry).
• Overload resolution is unchanged for existing shapes: single-entity and typed single-root overloads keep winning one-argument calls in both Java and Kotlin, Iterable arguments keep hitting the canonical methods, and an empty vararg call is a documented no-op.
• The write-sets reference now uses the vararg shape wherever entities are enumerated inline (the collection examples keep the Iterable form), matching the comparison pages shipped in docs: compare JPA cascades with Storm write sets (reference + tutorial) #272.

Verification

• WriteSetIntegrationTest (storm-core): 25 existing tests plus vararg insert/update/remove round trip and empty-call no-ops (27 total, green).
• WriteSetTest (storm-kotlin): vararg resolution exercised from Kotlin (7 total, green).

Fixes #273