feat(webui): minify and gzip-compress the embedded web assets

Two build-time steps shrink the embedded Web UI (html/) from 113,015 to
34,784 bytes (30.8 %):

- tools/minify.py (new): string-literal-safe minifier that strips
  comments and indentation; comments are pure overhead on the wire, so
  the transfer size drops already before gzip (113,015 -> 101,498).
  Binary files (.ico) are copied through unchanged.  The output goes to
  output/html_min/, the new fileadder input.
- fileadder -z: gzip-compresses every added file (zlib gzip format,
  Z_BEST_COMPRESSION); the generated f_data entries carry a gzip flag.
  tools/Makefile links the tool against -lz.
- httpd: adds "Content-Encoding: gzip" for flagged entries; the 8051
  only copies compressed bytes, decompression happens in the browser.
  The JSON API stays uncompressed.

Fixes required to make the sizes deterministic:
- fileadder terminated files at data_read + 1, so the strlen()-based
  size computation depended on uninitialised buffer content; the NUL
  now lands directly after the content.
- version.h was .PHONY, so every build rewrote it while parallel builds
  could read a half-written copy.
- the file list was unscoped and missed .css/.ico; it is now scoped to
  html/ and covers all embedded types.

Measured with make MACHINE=KP_9000_6XHML_X2: data block end moves from
0x5b977 to 0x487e0 (~78 KB freed on a 512 KB image).  Verified with
MACHINE=PCB_K0402WS_V3 and on real hardware (PCB-K0402WS-V3).

Makefile

@@ -75,10 +75,23 @@ SRCS += \
 
 OBJS = ${SRCS:%.c=$(BUILDDIR)/%.rel}
 DEPS := ${SRCS:%.c=$(BUILDDIR)/%.d}
-HTML := $(shell find $(html) -name '*.js' -or -name '*.html' -or -name '*.svg')
+HTML := $(shell find html -name '*.js' -or -name '*.html' -or -name '*.svg' -or -name '*.css' -or -name '*.ico')
 
-html_data.c html_data.h: $(HTML) tools/output/fileadder
-	tools/output/fileadder -a $(HTML_LOCATION) -s $(IMAGESIZE) -b BANK1 -d html -p html_data
+# Minified copy of the web UI sources, used as the fileadder input.
+# The raw html/ sources stay untouched for development; the minified
+# copy is a build artifact under output/.
+HTML_MIN := output/html_min
+.PHONY: html_min
+html_min: $(HTML)
+	rm -rf $(HTML_MIN)
+	mkdir -p $(HTML_MIN)
+	@for f in $(HTML); do python3 tools/minify.py $$f $(HTML_MIN)/$$(basename $$f) || exit 1; done
+
+html_data.c html_data.h: $(HTML) tools/output/fileadder html_min
+	tools/output/fileadder -a $(HTML_LOCATION) -s $(IMAGESIZE) -b BANK1 -z -d $(HTML_MIN) -p html_data
+# httpd.c includes html_data.h; declare it explicitly so a parallel
+# build (-j) does not compile httpd before the header is generated.
+$(BUILDDIR)/httpd/httpd.rel: html_data.h
 
 $(VERSION_HEADER):
 	@echo "#ifndef VERSION_H" > $(VERSION_HEADER)
@@ -118,11 +131,11 @@ $(BUILDDIR)/rtlplayground-$(FILENAME_EXTENSION).bin: $(BUILDDIR)/rtlplayground.i
 	tools/output/imagebuilder -i $^ $@
 	tools/output/fileadder -a $(DEFAULT_CONFIG_LOCATION) -s $(IMAGESIZE) -d config.txt $@
 	tools/output/fileadder -a $(CONFIG_LOCATION) -s $(IMAGESIZE) -d config.txt $@
-	tools/output/fileadder -a $(HTML_LOCATION) -s $(IMAGESIZE) -d html -p html_data -b BANK1 $@
+	tools/output/fileadder -a $(HTML_LOCATION) -s $(IMAGESIZE) -z -d $(HTML_MIN) -p html_data -b BANK1 $@
 	tools/output/crc_calculator -u $@
 	ln -sf $(MACHINE)/rtlplayground-$(FILENAME_EXTENSION).bin output/rtlplayground.bin
 
-.PHONY: clean all $(SUBDIRS) $(VERSION_HEADER)
+.PHONY: clean all $(SUBDIRS)
 
 .PHONY:
 machine_check: