[tmm]

Decision 3, Swift half: JS's inspector, query metrics, and slowMaterializeThreshold in zero-swift, lined up with the Rust port (next PR). Stacked on #206; zero-swift patch changeset, as earlier Swift feature changesets are.

var options = ZeroOptions(...); options.slowMaterializeThreshold = 5_000   // JS default
let insp = zero.inspector
try await insp.metrics()                       // client digests merged with the server's
try await insp.client.queries()                // ZQL, row counts, hydrate/update timings
try await insp.clientsWithQueries()
try await insp.serverVersion()
try await insp.analyzeQuery(request, options: .init(joinPlans: true))
try await insp.authenticate(password: "...")

Same shape as Rust's zero.inspector(): metrics, client/clientGroup (queries, map, rows, clients, clientsWithQueries), serverVersion, analyzeQuery / analyzeServerAST / analyzeNamedQuery, authenticate, formatPlannerEvents; Metrics of five TDigests; typed InspectorQuery and AnalyzeQueryResult.

How it differs from JS, as Rust does:

• No password dialog: a refused request throws InspectorError.authenticationRequired; authenticate(password:) remembers an accepted password and presents it again after a reconnect, in JS's wire order.
• Pending requests fail on a dropped connection or close() instead of waiting forever.
• The slow-materialization log is one line, <message> <queryID> <AST JSON> <ms>.
• TDigest matches JS bit for bit: V8's fdlibm asin/sin are ported, since glibc's differ in the last bit and move centroid splits (checked on 846,739 inputs and 400 seeded JS digests).

Metrics are recorded where JS calls addMetric; updates are sampled 1 in 32 and weighted 32 (UPDATE_SAMPLE_RATE).

Tests: TDigestTests (7) and InspectorTests (13), with expected values from running the repo's JS sources. Locally: swift format lint --strict, ZeroTests 160 passed, ConformanceTests 63 passed, docs build, and scripts/api-breaks.sh: no public API breaks since 0.7.12. The Rust PR adds the shared JS vectors (client-inspector.json, tdigest.json, ast-to-zql.json, and logs in two existing ones), and Swift's conformance runner replays them there.