console: show Materialize's execution time next to the round trip

[ggevay]

Draft, 100% Claude

Motivation

Part of CNS-170. The SQL Shell shows "Returned in X", a round trip measured in the browser. On a slow network, a query served from an index looks as slow as one that runs a dataflow. Materialize can report its own execution time per statement (the emit_execution_time_notice session variable, separate PR). The Shell now shows it next to the round trip.

Description

• Opt-in.
  • The Shell sets emit_execution_time_notice=on when it connects.
  • An environment that does not know the variable answers with a startup notice ("unrecognized configuration parameter"). The Shell hides any such failure to apply the variable and shows the round trip alone.
• Notice handling.
  • A notice with code MZ012 is stored as the result's execution time instead of being listed with the result's notices.
  • The server sends it before the statement's CommandComplete, so it lands on the statement it describes.
  • An MZ012 notice the Shell cannot parse, for example of a kind added later, is dropped, which leaves the round trip alone.
  • Fixes a typo in the state machine: a notice during a statement without rows no longer moves it to the state for statements with rows.
• Display.
  • Example: Returned in 148.0ms · 12.0ms to first row (served from an index).
  • The label depends on the kind: to first row, to empty result, in Materialize, before or including commit.
  • The execution strategy is named for reads.
  • Times below 0.1 ms show as <0.1ms.
  • The tooltip explains both numbers.
• No feature flag. The variable is opt-in on the server, and the Shell ignores environments without it. Reverting this change is the off switch.
• Docs. sql-shell.md gains a "Query timing" section.

User-visible effect: below each result, the SQL Shell shows Materialize's own execution time next to the round trip, on environments that report it.

Verification

• executionTime.test.ts (new): parsing, every kind's label, unknown strategies, sub-0.1 ms times, and the filter for a failed startup setting.
• webSocketFsm.test.ts: an MZ012 notice becomes the latest result's execution time and is not listed as a notice. An MZ012 notice whose detail does not parse is dropped.

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