refactor(erd): ErdSource per connection, schema-aware catalog, neighbourhood load (#1147)

[ZhuchkaTriplesix]

Refs #1147

Changes

• ErdSource with loadSchema() and loadNeighbourhood(table, depth). SqlErdSource implements it on a query runner for PostgreSQL, MySQL and SQLite.
• PostgreSQL catalog: tables outside the current schema are named schema.table, and foreign keys are read across user schemas, so a reference into another schema is drawn with the same key as the rest of the diagram.
• Neighbourhood: foreign keys walked both ways up to depth, cycles and self references end the walk, columns are loaded only for the tables found. Pure function ErdCatalog.neighbourhood.
• ErdView takes an optional source and focusTable (picked and centred after the first draw). Without a source it works as before.
• GenericSqlWorkspace.showDiagram (default on); ExtensionSqlWorkspace turns it off, so the extension SQL workspace has no Diagram button.
• Tests: neighbourhood depth, cycles, self references, a table without relations, the neighbourhood load with a fake runner, PostgreSQL cross-schema SQL.

Not in this PR

• The table browser does not use its own read-only session yet (fix(postgres): do not share the SQL-editor read-write socket with Table Browser #785, fix(sqlite): do not share the SQL-editor Database with Table Browser #794, fix(mysql): do not share the SQL-editor socket with Table Browser #802): the source is the diagram's delegate for now.
• No ErdTableRef type: names are strings in the diagram's naming, and the schema is part of the name (sales.orders) rather than a separate field.
• PostgreSQL cross-schema behaviour is covered by SQL string checks; it has not been run against a live database here.
• The table browser's Relations view (feat(erd): Relations view in the table browser #1149) is the next step on top of this.

🤖 Generated with Claude Code

[github-actions]

Grid scroll benchmark

metric	base	PR	change
p50	7.81 ms	13.67 ms	+75.0% ⚠️
p90	18.53 ms	20.01 ms	+8.0% ⚠️
p99	36.17 ms	44.80 ms	+23.9% ⚠️
stutters	334.00	326.00	-2.4%

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	12.13 ms	16.73 ms	+37.9% ⚠️
p90	21.87 ms	24.22 ms	+10.7% ⚠️
p99	47.89 ms	53.24 ms	+11.2% ⚠️
stutters	288.00	279.00	-3.1%

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