MQTT: intern per-subscription topic strings

## Problem

Minor: each MQTT subscription retains ~3 fresh copies of its topic string (`Msg/topicN`, `/topicN`, `topicN` shapes measured at 15,000 copies each in a 15k-subscription local heap snapshot, ~1.1 MB) even when the workload has few distinct topics (29 in the measured production case). Topic strings arrive from packet parsing, so each subscribe allocates new strings; `DurableSubscriptionsSession` then retains them per subscription (`subscription.topic`, session records, contexts).

## Proposal

Intern the topic string once per session (or via a small bounded map) at `addSubscription` so duplicate subscriptions share one string. ~1 KB/connection at 15 subs/conn; trivial but only worth doing alongside other work in this file.

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_Filed by an AI agent (Claude Code) from a subscription-path memory/CPU investigation._

Start at DurableSubscriptionsSession.addSubscription and trace how subscription.topic, session records, and contexts retain parsed topic strings. Confirm the current allocation and retention behavior, then verify that duplicate topics within a session share storage without changing subscription behavior or exceeding the intended memory bounds.

Written by the indexing model from the issue text.