Install daedalus-decoder.pc for sibling consumers

Adds pkg-config plumbing so consumers (daedalus-v4l2 daemon for the
upcoming PR-Q3a shadow-mode wiring; the daedalus_decode_h264 CLI when
built outside this tree) can locate libdaedalus_decoder.a + the public
header via pkg_check_modules / pkg-config.

Mirrors daedalus-fourier's relocatable-prefix scheme: prefix is derived
from ${pcfiledir} so cmake --install --prefix /foo produces a .pc that
resolves to /foo at lookup time. Verified across two install prefixes.

daedalus-fourier is declared as a public Requires: because consumers
static-linking libdaedalus_decoder.a also need libdaedalus_core.a in
their link line to resolve the daedalus_ctx_* / daedalus_recipe_*
symbols this archive references.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -237,12 +237,48 @@ endif()
 
 # ---- Install ------------------------------------------------------
 #
-# Library + public header.  Stage 2/3 will add a pkg-config file and
+# Installs:
-# CMake config exports once the API stabilises; pre-0.1 the scaffold
+#   - libdaedalus_decoder.a       → ${CMAKE_INSTALL_LIBDIR}
-# install just gives the static archive a home.
+#   - include/daedalus_decoder.h  → ${CMAKE_INSTALL_INCLUDEDIR}
+#   - daedalus-decoder.pc         → ${CMAKE_INSTALL_LIBDIR}/pkgconfig
+#
+# The .pc lets sibling consumers (daedalus-v4l2 daemon, the
+# daedalus_decode_h264 CLI when built externally) discover the static
+# archive + headers via pkg-config.  daedalus-fourier is declared as a
+# public `Requires:` because the consumer (which static-links
+# libdaedalus_decoder.a) also needs daedalus-fourier in its own link
+# line to resolve the daedalus_ctx_* / daedalus_recipe_* symbols this
+# archive references.
+#
+# Relocatable-prefix scheme mirrors daedalus-fourier's .pc generation:
+# `prefix` is derived from ${pcfiledir} so `cmake --install --prefix /foo`
+# produces a .pc that resolves prefix=/foo at lookup time, regardless of
+# what CMAKE_INSTALL_PREFIX was at configure time.
 
 include(GNUInstallDirs)
 install(TARGETS daedalus_decoder
     ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR})
 install(FILES include/daedalus_decoder.h
     DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+
+file(RELATIVE_PATH PKGCONFIG_PCDIR_TO_PREFIX
+    "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}/pkgconfig"
+    "${CMAKE_INSTALL_PREFIX}")
+
+set(PKGCONFIG_OUT ${CMAKE_CURRENT_BINARY_DIR}/daedalus-decoder.pc)
+file(WRITE ${PKGCONFIG_OUT}
+"prefix=\${pcfiledir}/${PKGCONFIG_PCDIR_TO_PREFIX}
+exec_prefix=\${prefix}
+libdir=\${prefix}/${CMAKE_INSTALL_LIBDIR}
+includedir=\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}
+
+Name: daedalus-decoder
+Description: Frame-major H.264 decoder on V3D7 via daedalus-fourier primitives
+Version: ${PROJECT_VERSION}
+Libs: -L\${libdir} -ldaedalus_decoder
+Requires: daedalus-fourier
+Cflags: -I\${includedir}
+")
+install(FILES ${PKGCONFIG_OUT}
+    DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig
+)