feat(table): Data | Relations view for tables (#1149)

[ZhuchkaTriplesix]

Refs #1149

Changes

• Table toolbar: a Data | Relations switch for tables of PostgreSQL, MySQL and SQLite. Views, materialized views and extension tables get no switch.
• Relations draws the table's neighbourhood with the table focused and centred, using the table browser's own read-only session through TableDataSqlAdapter (a SqlExecutionDelegate over loadCustomSql).
• Depth 1 / 2 buttons; the neighbourhood loads when first opened and stays until the view is rebuilt.
• The grid stays alive in an IndexedStack, so staged edits, page and filter survive Data → Relations → Data.
• A table without foreign keys shows No foreign keys to or from <table> with Open full diagram.
• Double click on a neighbour calls onOpenNeighbour.
• Tests: the switch on tables, not on views, not on extension tables, and the Relations view opening with depth switching.

Not in this PR

• Double click on a neighbour is wired to the callback only; the table browser's callers do not pass one yet, so the neighbour does not open the browser. The Quick Switcher path (onOpenSchemaObject) still needs to be connected.
• The ERD's own toolbar (Auto layout, Export, Refresh) still shows below the depth row: two toolbar rows instead of one.
• Command palette entries Table: Show relations / Table: Show data are not added.
• The staged-edits-survive check is not covered by a widget test yet.

🤖 Generated with Claude Code

[github-actions]

Grid scroll benchmark

metric	base	PR	change
p50	17.34 ms	14.12 ms	-18.6%
p90	23.80 ms	25.53 ms	+7.3% ⚠️
p99	47.07 ms	56.36 ms	+19.7% ⚠️
stutters	284.00	270.00	-4.9%

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

[github-actions]

Grid scroll benchmark

metric	base	PR	change
p50	6.73 ms	7.40 ms	+10.0% ⚠️
p90	14.79 ms	16.98 ms	+14.8% ⚠️
p99	29.79 ms	30.98 ms	+4.0%
stutters	341.00	334.00	-2.1%

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