Add two-layer GraphQL schema drift checker

[dkijania]

Stacked on #5 — review/merge that one first; rebase this on `main` afterward.

Summary

The JS SDK previously had no schema-drift detection. This adds parity with the Go / Python / Rust SDKs:

1. Introspection diff (Layer 1): compare `schema/graphql_schema.json` (new snapshot from devnet) to the live `__schema`. Detects added / removed types, field-type changes, argument-type changes (e.g. `UInt64 → TokenId` — the exact bug fixed in Fix QUERY_ACCOUNT_WITH_TOKEN: token arg type is TokenId, not UInt64 #5), inputField-type changes, enum-value changes.
2. Live query check (Layer 2): parse `src/queries.ts`, send each operation with sentinel variables, classify errors as schema drift vs runtime. Drift patterns matched against the message before the path-based runtime fallback (Mina attaches `path` to many validation errors). Bare `"expected type"` deliberately excluded — too many FPs on value coercion; `"expected on field X, found Y"` routed to runtime explicitly.

Design choices

• `AbortController` with 30 s timeout on every `fetch`
• HTTP status check (non-2xx → error with body snippet)
• `normalizeSchema` throws on error envelopes (no silent fake-empty)
• `nameOf` sort key safe against `null` / non-object entries
• `kind` nil-guard avoids spurious ` -> X` diffs
• `parseQueries` filters to bodies that start with an operation keyword
• `parseVariableDecls` anchored to body start
• Sentinel table supports `SignatureInput` (legitimate `null`) via a `NO_SENTINEL` symbol that distinguishes "no sentinel known" from "explicit null sentinel"
• Connection / HTTP errors → infra failures bucket; always exit 1
• `SKIP`ped ops (stale sentinel table) fail in `--strict`

CI

`.github/workflows/schema-drift.yml` mirrors the other SDKs: master / compatible / develop matrix against lightnet images; weekly cron + `workflow_dispatch` + PRs that touch `schema/` or the script. Exposes `npm run check:drift`.

Test plan

• Clean run against `compatible-latest-lightnet` in `--strict`: exit 0, 13 ok, 0 drift, 4 runtime (QUERY_BLOCK / QUERY_TRANSACTION_STATUS hit application-level constraints; correctly classified as runtime), 0 skipped, 0 infra-failures.
• Same negative cases as the Go / Python / Rust PRs (arg-type drift, inputField-type drift, unknown-field, unknown input type, unreachable port) all behave as expected.

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