build(pglite): cap max_wal_size at 128MB in the wasm module

PGlite splices a fixed list of -c settings into every start and never
reads postgresql.conf or postgresql.auto.conf on restart (ALTER SYSTEM
writes to the wrong directory and pg_reload_conf is a no-op without a
postmaster), so the only way to lower max_wal_size is that list. With
128MB, a CHECKPOINT trims pg_wal from 405 MB to 134 MB locally instead
of letting it grow to the 1 GB default.

db/pglite/build/README.md

@@ -11,6 +11,11 @@ electric-sql/postgres-pglite, branch REL_17_5_WASM-pglite). Clone it to /tmp/pgl
 - `wasi-c.diff` — makes the forced `-fPIC` in the compiler wrapper overridable (`PGLITE_PIC`). NOTE: the matching
   `-fpic` in `postgresql-src/wasm-build/build-pgcore.sh` (cloned at build time) is not covered by a diff here; the
   `-fno-pic` experiment did not take effect and is unmeasured.
+- `max_wal_size.diff` — goes in `patches/postgresql-pglite/` as `src-include-port-wasm_common.h.zz-max-wal-size.diff`
+  (it must sort after pglite4j's own `wasm_common.h` patch). PGlite splices a fixed list of `-c` settings into
+  every start (`WASM_PGOPTS`), never reads `postgresql.conf`/`postgresql.auto.conf` on restart, and `pg_reload_conf()`
+  is a no-op without a postmaster, so `ALTER SYSTEM` cannot change anything. Adding `max_wal_size=128MB` to that
+  list caps `pg_wal` at ~130 MB instead of 1 GB once a `CHECKPOINT` runs.
 - `message_context_reset.diff` — goes in `patches/pglite-wasm/`. The fork stubs `MemoryContextResetAndDeleteChildren`
   (removed in PostgreSQL 17) as an empty macro, so `MessageContext` is never reset and every wire message leaks its
   parse/plan state (~10 KB per statement; a library scan reached 15 GB). The patch maps it to `MemoryContextReset`.

db/pglite/build/max_wal_size.diff

@@ -0,0 +1,9 @@
+--- a/src/include/port/wasm_common.h
++++ b/src/include/port/wasm_common.h
+@@ -63,5 +63,6 @@
+         "-c", "synchronous_commit=on",\
+         "-c", "wal_buffers=4MB",\
+         "-c", "min_wal_size=80MB",\
++        "-c", "max_wal_size=128MB",\
+         "-c", "shared_buffers=128MB"
+