fix(mongodb-mcp): add BsonFieldType enum to Zig FFI (standards#155) (#119)

## Summary

Class C ABI gap from the Phase 2 full-tree survey (standards#92):
`MongodbMcp.SafeDatabase.BsonFieldType` (11 BSON wire-protocol field
types) was declared in the Idris2 ABI but absent from
`mongodb_mcp_ffi.zig`. Adds the matching `pub const BsonFieldType =
enum(c_int)` with the canonical encoding (1–18 with BSON-spec gaps).

ABI parity only — the wire-protocol encoder/decoder that *uses* these
tags is a separate piece of work; declaring the enum here lets
`iseriser abi-verify` structurally check the encoding.

## Verified

- `iseriser abi-verify` → exit 0 (3 enums clean)
- `zig build test` → 17/17 green

## Next

After merge, mongodb-mcp can join the abi-drift allowlist
(boj-server#115's 56 → 57).

Closes hyperpolymath/standards#155
Refs hyperpolymath/standards#92
Refs hyperpolymath/standards#89

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

cartridges/mongodb-mcp/ffi/mongodb_mcp_ffi.zig

@@ -43,6 +43,27 @@ pub const MongodbAction = enum(c_int) {
     list_databases = 15,
 };
 
+/// BSON wire-protocol field type tags. Matches Idris2
+/// `MongodbMcp.SafeDatabase.BsonFieldType` + `bsonFieldTypeToInt`.
+/// Integer codes follow the BSON spec — gaps (6, 11–15, 17) are by
+/// design (reserved / deprecated BSON codes Idris2 does not expose).
+/// Declared here so `iseriser abi-verify` can structurally check the
+/// encoding against the Idris2 source; the wire-protocol encoder will
+/// use these values when introduced.
+pub const BsonFieldType = enum(c_int) {
+    bson_double = 1,
+    bson_string = 2,
+    bson_document = 3,
+    bson_array = 4,
+    bson_binary = 5,
+    bson_object_id = 7,
+    bson_bool = 8,
+    bson_date_time = 9,
+    bson_null = 10,
+    bson_int32 = 16,
+    bson_int64 = 18,
+};
+
 /// Validate a state transition against the proven Idris2 transition graph.
 fn isValidTransition(from: ConnState, to: ConnState) bool {
     return switch (from) {