perf(sql-workspace): rows show before the table schema lookup, schema cache (#1171)

[ZhuchkaTriplesix]

Refs #1171

Changes

• After a query the rows show at once. The table schema (primary keys, column types, edit hint, save state) is applied when it arrives, from a session cache or from the delegate in the background. An answer for a result the tab has already left is dropped.
• The cache is keyed by the statement and its result columns. It is cleared after any statement that is not a read (select, with, values, show, explain, pragma, describe), so a table changed by DDL is looked up again.
• Tests: rows show while the schema lookup never completes; the same query is not looked up twice.

Not in this PR

• Clearing the cache on Refresh in the connection tree: the workspace does not reach the tree. The cache also goes away with the tab.
• The DDL-clears-cache path has no test yet; it needs a way to change the editor text in a widget test.
• Background resolution for MySQL and SQLite is shared code in the workspace, so it applies there too; the dialect-specific lookups are not measured.

[github-actions]

Grid scroll benchmark

metric	base	PR	change
p50	16.95 ms	16.54 ms	-2.4%
p90	24.82 ms	29.47 ms	+18.7% ⚠️
p99	54.40 ms	57.83 ms	+6.3% ⚠️
stutters	271.00	264.00	-2.6%

Informational only (threshold 5%). Shared CI runners are noisy; re-run before trusting a single result.