feat(dirty-content-fingerprint): xceadmin build-info module gains fingerprint

Threads dirty_fingerprint through as an 8th positional arg from
xceadmin/meson.build's custom_target into meson-build.sh, which adds
DIRTY_FINGERPRINT to the generated _build_info.py with an independent
empty-string fallback (not "unknown", matching this feature's convention
everywhere else). GIT_COMMIT/BUILD_TIME's existing generation and
"unknown" fallback are completely untouched.

Verified independently by review across three full real build+test+wheel
cycles: dirty tree produces a real, non-empty fingerprint present
identically in the mirrored tree, build/lib copy, an installed venv, and
the packaged wheel, cross-checked byte-for-byte against that same build's
compile_commands.json; explicit-empty fingerprint with real commit/time
resolves DIRTY_FINGERPRINT to genuinely empty; all-empty scenario confirms
the two fallback conventions ("unknown" vs empty) coexist without
interference. 261 passed/3 skipped throughout.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/xceadmin/meson.build

@@ -50,6 +50,14 @@ xceadmin_stamp = custom_target('xceadmin',
     # XCE_GIT_COMMIT/XCE_BUILD_TIME handling.
     get_option('commit_id'),
     get_option('build_time'),
+    # dirty-content-fingerprint task 5.1: third positional arg, same
+    # resolved-option-value style as commit_id/build_time above. Unlike
+    # those two, its meson-build.sh-side fallback on empty is a plain empty
+    # string, NOT "unknown" -- matching this feature's convention
+    # everywhere else (go_build's XCE_DIRTY_FINGERPRINT, the C daemons'
+    # dirty_fingerprint), not the original feature's commit_id/build_time
+    # convention.
+    get_option('dirty_fingerprint'),
   ],
   build_by_default: true,
   build_always_stale: true,