xConnect for virtual tables to query core db connection

[PThorpe92]

Re-Opening #1076 because it had bit-rotted to a point of no return.

However it has improved. Now with Weak references and no incrementing Rc strong counts.

This also includes a better test extension that returns info about the other tables in the schema.

(theme doesn't show rows column)

[penberg]

This pull request has been marked as stale due to inactivity. It will be closed in 7 days if no further activity occurs.

[Copilot]

Pull Request Overview

This PR updates the virtual table API to pass a live database connection into extensions, adds a new TableStats virtual table with tests for row counts, and revises the registration macros and core wiring to use weak references rather than bumping Rc strong counts.

• Extend all VTabModule::open functions to accept an optional Connection handle.
• Implement connect in the core so extensions get a weak-backed Conn pointer.
• Add a new tablestats virtual table module and CLI tests for it.

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
testing/cli_tests/extensions.py	Added new test_tablestats and adjusted TestLimboShell callsite.
macros/src/lib.rs	Updated generated FFI signatures to pass conn and register it.
extensions/tests/src/lib.rs	Added TableStats module implementation and derive boilerplate.
extensions/series/src/lib.rs	Updated GenerateSeriesVTab::open signature and tests.
extensions/core/src/vtabs.rs	Added conn field and new FFI types, updated VTabModule trait.
extensions/core/src/types.rs	Introduced new ResultCode variants and StepResult.
extensions/core/src/lib.rs	Exposed the new StepResult and core Connection API.
extensions/core/README.md	Updated example to show passing Connection into open.
extensions/completion/src/lib.rs	Updated CompletionVTab::open signature.
core/types.rs	Added OwnedValue::from_ffi_ptr helper.
core/ext/vtab_connect.rs	New FFI shims using weak Rc for extension connections.
core/ext/mod.rs & core/ext/dynamic.rs	Wire the new connect function into dynamic and built-in loading.
core/translate/main_loop.rs	Skip zero‐index constraints in best_index loop.
core/lib.rs	Updated VirtualTable::open to forward conn pointer.

Comments suppressed due to low confidence (2)

testing/cli_tests/extensions.py:502

• Other tests still call TestLimboShell() without arguments. If the constructor signature changed, those will break—unify all calls to use the new signature or overload the constructor.

limbo = TestLimboShell("")

extensions/core/README.md:195

• [nitpick] The example now mentions Rc<Connection> but does not show importing std::rc::Rc (or limbo_ext::Connection). Add those use statements to keep the snippet complete.

fn open(&self, conn: Option<Rc<Connection>>) -> Result<Self::VCursor, Self::Error> {