refactor(ast): converge dispatch tables onto a canonical node catalog (#1243)

[nickna]

Issue #1243 — Converge the AST dispatch tables onto a single canonical node list

Follow-up to #1143 / epic #1094.

Approach

The issue asked to "evaluate a source generator (or a single canonical table)" so a node can't
silently skip a dispatch table. I started down the source-generator path but that means introducing
Roslyn (Microsoft.CodeAnalysis.CSharp) as a build-time dependency — which this repo does not
currently use, on a preview SDK. A generator's only real advantage here is turning a missed node into
a compile error rather than a test failure, and only one table is actually hand-maintained
(the AstVisitorBase switch). NodeRegistry already derives its node set by reflection and validates
exhaustiveness at startup.

So this takes the lighter, dependency-free route: a single canonical table everything converges on.

What changed

• New Parsing/Visitors/AstNodeCatalog.cs — the single source of truth for the AST node set,
  derived once (reflection) from the Expr/Stmt records in AST.cs using the exact predicate the
  dispatch tables used before (concrete, directly-nested, inherits the base). Exposes ExprTypes /
  StmtTypes.

• NodeRegistry — Freeze(), FreezeAsync(), and AutoRegister() now read the node set from
  AstNodeCatalog instead of each re-running its own typeof(Expr).GetNestedTypes(...) reflection.
  Three duplicated reflection sites collapse to one. Behavior is identical (same predicate, same set),
  so the interpreter/type-checker startup exhaustiveness checks — and the InterpreterRegistry
  async-handler FreezeAsync check — are unchanged except for their source of truth.

• New SharpTS.Tests/RegistryTests/AstDispatchTests.cs — guards the one hand-maintained table.
  It walks every catalog node through AstVisitorBase.Visit and asserts none falls through to the
  default arm (the "no dispatch case" NotSupportedException). A self-check (Probe_detects_an_unhandled_node)
  proves the discriminator actually fires for an out-of-catalog node. Adding a node to AST.cs now
  auto-appears in the catalog, so forgetting its AstVisitorBase case fails this test instead of only
  blowing up at visit time.

Result — every dispatch table now converges on AstNodeCatalog

Table	How a missing node is caught
NodeRegistry sync handlers (interp + checker)	startup Freeze() against the catalog
NodeRegistry async expr handlers (interp)	startup FreezeAsync() against the catalog
AstVisitorBase.Visit(Expr/Stmt) switch	AstDispatchTests against the catalog

No new dependencies; no new projects.

Testing

• AstDispatchTests: 4/4 pass.
• Full xUnit suite: 15419 passed, 0 failed.
• Test262 (interp + compiled): 22 passed, 0 failed, 4 skipped (baseline-green).
• TypeScript conformance: 31 passed, 0 failed (baseline-green).
• Interpreter + compiled smoke of arrays/functions/classes: identical output in both modes.

Closes #1243.