fix(v18): use pg_query_scan_tokens for scan

[pyramation]

Summary

Merge after constructive-io/libpg_query chore/sync-18-latest → 18-constructive. This PR's CI clones 18-constructive, and it only builds once that merge lands.

Upstream libpg_query switched from protobuf-c to upb and removed protobuf/pg_query.pb-c.h. The full-API wasm_wrapper.c included that header in order to unpack pg_query_scan() output, so the v18 WASM build fails to compile against the synced fork.

The fix uses upstream's new protobuf-free API:

- PgQueryScanResult r = pg_query_scan(input);  /* + pg_query__scan_result__unpack */
+ PgQueryScanTokensResult r = pg_query_scan_tokens(input);  /* tokens[i].{start,end,token,keyword_kind} */

scan() JSON output stays the same: same fields, same simplified tokenName mapping. version now comes from PG_VERSION_NUM, which is the same value protobuf used to carry. Tests and README now expect 180006 (PG 18.6).

Verified locally: v18 pnpm build + pnpm test pass 92/92 when built from the synced fork branch.

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Requested by: @pyramation

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

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[tenki-reviewer]

Review complete. No blocking issues — approved ✅; 1 nitpick below.

🧹 Nitpicks (1) — 🟢 1 low

• 🟢 Vendored libpg_query.md still documents removed scan API (libpg_query.md) — The PR migrates the wrapper to pg_query_scan_tokens/PgQueryScanTokensResult (versions/18/src/wasm_wrapper.c:378), but the vendored API reference versions/18/libpg_query.md still documents the removed protobuf-based pg_query_scan with an unpack workflow (lines 52-77 and 247).

---

The change moves both C wrappers (templates/full and versions/18) to the new pg_query_scan_tokens API: protobuf unpacking is removed, PgQueryScanTokensResult fields are read directly as a struct array, cleanup switches to pg_query_free_scan_tokens_result, and the reported version now comes from the PG_VERSION_NUM build constant. The emitted JSON shape ({"version":N,"tokens":[...]}) is unchanged, so the TypeScript scan/scanSync consumers remain compatible. The test suite in versions/18/test/pg18.test.js was updated for the new parser version, and versions/18/README.md documents the version bump. One low-severity doc divergence remains: the vendored API reference still describes the removed scan API.

Files	Change
templates/full/wasm_wrapper.c, versions/18/src/wasm_wrapper.c	Migrated to pg_query_scan_tokens/pg_query_free_scan_tokens_result, dropping protobuf includes and unpack code; token loops now iterate scan_result->tokens as structs and version is emitted from PG_VERSION_NUM.
versions/18/README.md	Version bump documentation to the new parser build.
versions/18/test/pg18.test.js	Test expectations updated for the pg18 parser and scan token output.
versions/18/libpg_query.md (untouched)	Still documents the removed pg_query_scan protobuf workflow; flagged for follow-up.

_{Reviewed commit: cfff958}