compat-eslint: SyntheticLazyNode base — architectural boundary for synthetic intermediates

Synthetic intermediate classes (TSTypeAnnotation, ClassBody, ChainExpression
wrapper, etc.) opted out of map registration via a `registerInMaps=false`
constructor arg. Easy to forget — and a forgotten synthetic class produces
silently-wrong parents on bottom-up materialise because the wrapper-route
tables don't see it.

Replace the boolean param with a `SyntheticLazyNode` abstract base. The 14
synthetic-creating classes now `extend SyntheticLazyNode` instead of
`extend LazyNode`; the base overrides `_registersInMaps()` to return false.
Adding a 15th synthetic class is now a visible architectural act in code
review (extends a different base + needs a navigation-table entry). Grep
`extends SyntheticLazyNode` to find all current synthetic classes.

JSXOpeningElementNode is hybrid (synthetic only when wrapping a
JsxSelfClosingElement); keeps the LazyNode base but overrides
`_registersInMaps()` to dispatch on `this._ts.kind`. Drops the now-unneeded
`synthetic` constructor arg.

Also drops a dead `registerInMaps` param on JSXIdentifierNode (all 6 call
sites passed `true`) and stale comments referencing the removed flag.

Tested: predicate-coverage / lazy-estree / scope-compat / selector-analysis
/ ts-ast-scan / compat-pipeline + dogfood (107 rules × 30 files clean) +
Dify cold/warm bench (no regression vs master).

Author: johnsoncodehk