runtests: report total elapsed seconds in all report styles

The '### runtests DONE at <ts>' human line gains '(elapsed <N>s)', the
RUNTESTS_RESULT summary line gains a trailing elapsed_seconds=<N>
key=value, and the json report gains a top-level "elapsed_seconds"
field - matching the name already used by the watch-mode run-end event,
which now shares the same computed value. Additive only; the parity
tool reads named keys and is unaffected (verified on all three report
styles).

Assisted-by: harness=claude; primary-model=claude-fable-5; api-location=anthropic.com

src/tests/runtests.tcl

@@ -81,12 +81,13 @@ proc runtests_dict_get_default {dictvalue key default} {
     return $default
 }
 
-proc runtests_json_report {status tallydict file_summaries slowest_tests} {
+proc runtests_json_report {status tallydict file_summaries slowest_tests elapsed_seconds} {
     set files_failed [dict get $tallydict files_with_fails]
     set files_warned [dict get $tallydict files_with_warnings]
     set fields [list]
     lappend fields "\"runner\":\"src/tests/runtests.tcl\""
     lappend fields "\"status\":[runtests_json_string $status]"
+    lappend fields "\"elapsed_seconds\":$elapsed_seconds"
     lappend fields "\"total\":[dict get $tallydict total]"
     lappend fields "\"passed\":[dict get $tallydict passed]"
     lappend fields "\"skipped\":[dict get $tallydict skipped]"
@@ -127,8 +128,8 @@ proc runtests_status {tallydict} {
     return pass
 }
 
-proc runtests_emit_result_line {status tallydict file_summaries} {
-    puts stdout "RUNTESTS_RESULT status=$status total=[dict get $tallydict total] passed=[dict get $tallydict passed] skipped=[dict get $tallydict skipped] failed=[dict get $tallydict failed] warnings=[llength [dict get $tallydict files_with_warnings]] files=[llength $file_summaries] files_failed=[llength [dict get $tallydict files_with_fails]]"
+proc runtests_emit_result_line {status tallydict file_summaries elapsed_seconds} {
+    puts stdout "RUNTESTS_RESULT status=$status total=[dict get $tallydict total] passed=[dict get $tallydict passed] skipped=[dict get $tallydict skipped] failed=[dict get $tallydict failed] warnings=[llength [dict get $tallydict files_with_warnings]] files=[llength $file_summaries] files_failed=[llength [dict get $tallydict files_with_fails]] elapsed_seconds=$elapsed_seconds"
 }
 
 proc runtests_warning_summary {code message summaryline_detected} {
@@ -1255,18 +1256,19 @@ foreach testfile_relative $testfiles {
 }
 set status [runtests_status $tallydict]
 set slowest_tests [runtests_slowest_tests $all_passes $opt_slowest]
+set elapsed_seconds [expr {[clock seconds] - $ts_start}]
 if {$jobs_mode} {
     dict set runtests_phase_times processing_s [format %.1f [expr {([clock milliseconds] - $_phase_t2) / 1000.0}]]
     if {!$report_json_only} {
         puts stdout "jobs phase times: parallel [dict get $runtests_phase_times parallel_s]s, serial tail [dict get $runtests_phase_times serial_s]s, processing [dict get $runtests_phase_times processing_s]s"
     }
 }
-runtests_udptee_event run-end status $status total [dict get $tallydict total] passed [dict get $tallydict passed] skipped [dict get $tallydict skipped] failed [dict get $tallydict failed] files [llength $file_summaries] elapsed_seconds [expr {[clock seconds] - $ts_start}] {*}$runtests_phase_times
+runtests_udptee_event run-end status $status total [dict get $tallydict total] passed [dict get $tallydict passed] skipped [dict get $tallydict skipped] failed [dict get $tallydict failed] files [llength $file_summaries] elapsed_seconds $elapsed_seconds {*}$runtests_phase_times
 
 if {!$report_json_only} {
     puts stdout "## runtests summary"
     puts stdout ""
-    runtests_emit_result_line $status $tallydict $file_summaries
+    runtests_emit_result_line $status $tallydict $file_summaries $elapsed_seconds
     puts stdout ""
 }
 
@@ -1374,7 +1376,7 @@ if {$report_emit_json} {
         puts stdout ""
         puts stdout "```json"
     }
-    puts stdout [runtests_json_report $status $tallydict $file_summaries $slowest_tests]
+    puts stdout [runtests_json_report $status $tallydict $file_summaries $slowest_tests $elapsed_seconds]
     if {!$report_json_only} {
         puts stdout "```"
         puts stdout ""
@@ -1382,7 +1384,7 @@ if {$report_emit_json} {
 }
 
 if {!$report_json_only} {
-    puts stdout "### runtests DONE at [clock format [clock seconds] -format "%Y-%m-%d %H:%M:%S"]"
+    puts stdout "### runtests DONE at [clock format [clock seconds] -format "%Y-%m-%d %H:%M:%S"] (elapsed ${elapsed_seconds}s)"
     puts stdout ""
 }