[DO NOT MERGE] Characterize shell subscription fanout and query cost

[Mjones13]

This adds server-only shell subscription characterization at 1, 2, 8, and 32 clients, with buffered startup, live post-marker fanout, cancellation, failure cleanup, and an opt-in real projection-query probe seeded with 10,000 synthetic turns. It also registers the focused transport check group. Production subscriptions, queries, schemas, event order, and public contracts are unchanged.

The probe records actual SQL, query plans, result cardinality, and Effect elapsed time using a fresh in-memory database and synthetic repository resolver. Measurements are descriptive samples; they do not establish CPU cost, constant query complexity, latency causality, or a shared-publisher savings bound. No publisher, index, or deduplicator is activated.

Validation on the exact task head: normal hooks, server type checking, scoped lint, formatting, and all 11 characterization cases. The standard run retains the 10 regular cases; the history probe is explicitly enabled with JONES_SHELL_HISTORY_PROBE=1.

[DO NOT MERGE] Qualification is based on the integrated frozen canonical revision 8de693b103e6fb69e7a8b07775a9dbbab8d0f149. Canonical main subsequently moved to 14811831cc8ca090257d3b8ddd127a2841179c40; open #67 at 3a1bfef9c78b0ce8d5b199fb3096f8e8b029adba also changes server.test.ts. These overlaps are not dependencies. Refresh the relevant source comparison and qualification before any merge. This is a draft characterization checkpoint; merge and production optimization are outside this change.

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