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.
eraiza0816