Merge upstream/main into feat/webui-compression

Resolve the conflicts against the 41 upstream commits since 52692d0:

- Makefile: keep the upstream parallel-build fixes (grouped html_data
  target with order-only deps, create_build_dir, crtbank, deterministic
  BUILD_DATE, atomic version.h write) and keep the minify + gzip
  pipeline (-z, html_min, .css/.ico in the html file list) on top.
- html: l2.js stays removed; port the upstream L2 fixes into the
  single-page main.js (walkL2() with empty-reply and 4096-entry cap
  handling, 3 retries on error).  The L2 poll now re-schedules itself
  after each walk and stops when the section is hidden.
- httpd/httpd.c merged cleanly (gzip Content-Encoding alongside the
  upstream GET walk bound and counters.json port check).
- doc/webui-compression.md: describe the upstream version.h write.

Verified: KP_9000_6XHML_X2 and PCB_K0402WS_V3 build sequentially and
with make -j8; all 8 embedded files decompress byte-identically;
httpd_sim serves main.js/index.html and the L2 JSON endpoint.

doc/link_aggregation.md

@@ -92,7 +92,7 @@ The following shows the network configuration
 On _both_ switches create a LAG with ports 1 and 2 inside and the default hash algorithm which takes
 source and destination ports into account, e.g. just use the default:
 ```
-> lag 0 1 2
+> lag 1 1 2
 ```
 
 

doc/webui-compression.md

@@ -75,8 +75,10 @@ JSON polling of the visible section).
 - `fileadder` terminates the embedded files with a NUL directly after the
   content (previously at `data_read + 1`), so the `strlen()`-based size
   computation no longer depends on uninitialised buffer content.
-- `version.h` is no longer `.PHONY`; every build previously rewrote it
-  while a parallel build (`make -j`) could read a half-written copy.
+- `version.h` is now written by a single atomic `printf` (change from
+  upstream) instead of five separate `echo`s, so a parallel build
+  (`make -j`) can no longer read a half-written copy.  It stays in
+  `.PHONY` so it is regenerated on every build.
 
 ## Verification