Fix pg_durable documentation examples

[tjgreen42]

Problem

Testing the README and user-facing documentation examples found several result-substitution issues:

• The README quick example used a captured multi-row result as a scalar in an array context. A bare $var capture substitutes only the first column of the first row, so the generated SQL was invalid.
• USER_GUIDE.md had additional examples with the same scalar-vs-row-set issue, unqualified playground tables, duplicate unnamed substituted columns, and signal workflows that captured order/doc but referenced undefined $order_id/$doc_id.
• docs/spec-signals.md duplicated the signal capture-reference mistakes.
• The Azure HTTP domains smoke check comment claimed broader SQL validation than the script performs.

Fix

• Use $batch.* row-set expansion where examples pass captured row sets between steps.
• Keep runnable User Guide examples aligned with the playground schema from the appendix.
• Alias substituted dot-notation columns in the dot-notation example.
• Reference captured result columns with $order.id and $doc.id in signal examples.
• Narrow the smoke-check comment to match its actual offline validation.

Verification

Tested against pgrx-managed PostgreSQL 17 and 18 instances built from this PR branch:

• README quick example completed and updated all expected rows.
• Corrected finite User Guide examples completed, including dot notation, df.if_rows(), configurable ETL, row-set expansion, and signal timeout flows.
• docs/SCENARIOS.md copy-paste scenarios completed; the scheduled HTTP scenario was started, observed through its first insert, then cancelled.
• Example smoke checks passed for azure-functions, azure-http-domains, and invoice-approval.
• Local non-cloud portions of the Azure example SQL scripts ran successfully.
• Patched signal snippets in docs/spec-signals.md completed with expected state changes.

[pinodeca]

Thanks for catching this!

[pinodeca]

LGTM! Thanks for QA-testing our docs!