refactor(vnext): extract realm-neutral parser backend (#179)

## Summary

- extract module decoding, parser invocation, result validation, and
error normalization into an internal realm-neutral backend
- preserve Node-specific realm validation, module loading, cleanup,
parser authority, and private AST ownership in the existing adapter
- add exhaustive backend contract tests covering bounds, cancellation
checkpoints, retry caching, hostile values, and ambient neutrality
- document the host/engine responsibility boundary

No public export, session wiring, worker protocol, or user-visible API
is added.

## Validation

- 1,041 unit tests passed; 1 intentional expected failure
- vNext coverage: 98.54% statements, 97.06% branches, 100% functions,
98.53% lines
- new backend coverage: 100% statements, branches, functions, and lines
- source and test TypeScript checks
- oxlint and test-integrity checks
- Chromium browser tests
- exact packed-consumer smoke test
- isolated worker placement and bundle-budget harness
- demo production build
- parser adapter benchmark
- git diff check

## Independent review

Two independent adversarial reviewers approved exact commit 96bd2ce with
zero actionable findings.

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Extracted a new internal backend for `node-sql-parser` and rewired the
Node adapter to call it. No public API changes.

- **Refactors**
- New `node-sql-parser-backend`: decodes module, runs `astify`,
validates output, and normalizes errors; no Node/window/worker refs.
- Node adapter now only handles realm checks, `require` loading,
cleanup, parser authority, and private AST ownership; uses backend
outcomes.
- Unified backend outcomes; concurrent load de‑dup and retry cache;
enforces `MAX_NODE_SQL_PARSER_STATEMENT_LENGTH`.
- Accepts constructor/`default`/`module.exports`/named `Parser`; ignores
accessors/proxies and redacts private data.
- Added backend tests (bounds, cancellation, retry, hostile values,
ambient neutrality) and documented the host/engine boundary.

- **Bug Fixes**
- Validate AST array lengths, rejecting spoofed or non‑integer length
descriptors.

<sup>Written for commit 08a9d67.
Summary will update on new commits.</sup>

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Author: Light2Dark

docs/vnext/node-sql-parser-adapter.md

@@ -45,6 +45,14 @@ statement slice. Locations remain partial: PostgreSQL commonly omits root and
 identifier locations, while BigQuery provides broader but still incomplete
 coverage.
 
+Module-shape decoding, parser invocation, result validation, and error
+normalization live in a realm-neutral internal backend engine. That engine has
+no Node, browser-window, or worker dependency. The Node adapter remains
+responsible for realm validation, module loading, cleanup, parser authority,
+and private AST ownership. A browser worker can therefore reuse the same
+backend semantics without importing Node globals or weakening main-realm
+artifact authenticity.
+
 Loading the distributed bundles may write `NodeSQLParser` or `global` on a
 global object. The adapter rejects parser loads whenever `window` or `self`
 exists, or `global` does not resolve exactly to `globalThis`, before loading a