test: the JSON generators against the tables the CLI wrote

page_impl.c and rtl837x_port.c are linked together over the simulated
engine, so /vlan.json, /vlanlist and /l2.json are checked against what
vlan_create, pvid and the L2 table hold, through the same bytes the switch
would. The L2 walk is driven the way the web page drives it, page after
page until an index repeats, which is also where the repeat of the first
entry is pinned down as the wrap marker the page relies on. The counter
test fails on the code before the previous commit.

test/README.md

@@ -25,7 +25,7 @@ fires — so it drops straight into CI.
   fetches a MIB counter on `STAT_GET` and drops entries on `L2_TBL_FLUSH_CTRL`.
   Nothing ever reports busy, so a polling loop runs once. The header states the
   VLAN and L2 entry layouts independently of the firmware, so a test can hold
-  what `rtl837x_port.c` writes against an independent statement of the layout.
+  what `rtl837x_port.c` writes against what `page_impl.c` reads.
 - **`env_tables.c`** carries the globals and leaf calls those two modules link
   against; **`stub/`** stands in for the two headers only a firmware build
   generates, so the harness needs no SDCC build first.
@@ -46,6 +46,7 @@ fires — so it drops straight into CI.
 |-------------|---------------|--------------------|
 | `test_cmd_editor` | `cmd_editor.c` | **C4** — full-line hang + `cmd_buffer` 1-byte overflow; basic entry & backspace regressions |
 | `test_port_tables` | `rtl837x_port.c` | VLAN entry layout and round trip, PVID register sharing, static multicast and management entries, per-port flush, trunk membership and hash seed |
+| `test_page_json` | `httpd/page_impl.c` + `rtl837x_port.c` | `/vlan.json`, `/vlanlist`, `/l2.json` (walk, wrap marker, paging inside `outbuf`), 64-bit counters in `/status.json` and `/counters.json` |
 
 ## Adding a test for another module
 1. Write `test_<module>.c` with `main()` driving the module's entry points and