runtests: native Tcl 8.6 support via guarded punk::lib compat load (lpop)

The validated compat family is brought across for the harness: guarded
'package require punk::lib' when ::lpop is absent, at both boot levels -
the toplevel two-stage boot (right after punk::args, whose parse needs
lpop at runtests.tcl's own argument parsing) and the singleproc
testinterp (before the shellrun preload, whose runx parses via
punk::args::parse). No-ops on Tcl 8.7+/9: -discover-only stays ~0.35s.

Native 8.6.17 verification: -discover-only works; punkcheck subtree
84/84, punk/lib subtree 43/43+1 gated skip, runner discovery suite
22/22, and a multiproc (-singleproc 0) probe 49/49.

Tcl 9.0.3 regression per the src/tests/AGENTS.md runner-change
contract: full suite in both modes with -report json,
runtests_parity.tcl reports PARITY: ok (files=96 total=1064
passed=1047 skipped=16 failed=1 - the sole failure is the documented
pre-existing exec-14.3 baseline).

src/tests/AGENTS.md records the supported-interpreter change and the
still-broken punk86-kit runtests route (kit/dev module skew - use a
native 8.6 tclsh for 8.6 legs).

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

src/tests/AGENTS.md

@@ -41,6 +41,7 @@ Top-level test harness and source-tree tests for ShellSpy/Punk. Tests here exerc
 ## Work Guidance
 
 - Select an appropriate interpreter, such as `tclsh`, `tclsh90`, or a built Punk executable such as `punk902z`.
+- Native Tcl 8.6 tclsh is a supported runner interpreter (2026-07-21): `runtests.tcl` loads punk::lib's validated forward-compat command family (lpop etc - builtin parity pinned by `modules/punk/lib/testsuites/lib/compat.test`) at both boot levels (toplevel and testinterp) when the builtins are absent. The guards are no-ops on Tcl 8.7+/9, preserving the G-093 fast two-stage `-discover-only` boot. The punk86-kit route for runtests remains broken (kit/dev module skew loading dev punk::path) - use a native 8.6 tclsh for 8.6 legs.
 - Run the full source-tree suite with `<tcl_interpreter> src/tests/runtests.tcl`.
 - Use `-include-paths` with directory glob patterns relative to `src/tests/`, forward slashes — `modules/punk/args/***` runs that whole subtree (`X/***` deliberately excludes files directly in `X`; bare `X` is files-directly-in-X only). The flag accepts a space-separated list of patterns and may also be repeated, with all occurrences accumulating (`-multiple 1` as of 2026-07-10; previously the last flag silently won). Single-file targeting: `-include-paths modules/punk/args/testsuites/args dynamic.test`.
 - Use `-exclude-paths` (same syntax and accumulation) to remove directories from discovery — `**`|`***` tails prune the matched subtree, other matches drop only that directory's files, and exclusion wins for the same file. It gives broad runs a middle ground between full and targeted: e.g `-exclude-paths shell/***` when no built kit is present, or skipping a heavy suite family unrelated to the change.

src/tests/runtests.tcl

@@ -39,6 +39,14 @@ if {$libdir_arch ni $::auto_path} {
 #inspection tool (boot ~1s instead of ~11s) for humans, agents and the runner-targeting
 #regression suite (runner/testsuites/discovery/pathdiscovery.test).
 package require punk::args
+if {[info commands ::lpop] eq ""} {
+    #Native Tcl 8.6 interpreter: no builtin lpop, which punk::args::parse (and later runner code)
+    #uses. punk::lib installs its validated forward-compat family (lpop, ledit, lremove, ...) as
+    #global aliases whenever the builtins are absent - behavioural parity with the 8.7/9 builtins
+    #is pinned by modules/punk/lib/testsuites/lib/compat.test. Guarded: on Tcl 8.7+/9 this never
+    #runs, preserving the G-093 fast two-stage boot for -discover-only.
+    package require punk::lib
+}
 
 #test-file discovery + -include-paths/-exclude-paths filtering (G-093) - shared with the
 #runner-targeting regression suite
@@ -1150,6 +1158,14 @@ foreach testfile_relative $testfiles {
         testinterp eval [list set ::argc [llength $tcltestoptions]]
         testinterp eval {package require tcltest}
         testinterp eval [list tcltest::configure {*}$tcltestoptions]
+        testinterp eval {
+            if {[info commands ::lpop] eq ""} {
+                #native Tcl 8.6 testinterp: shellrun's runx (the runner's capture mechanism) parses via
+                #punk::args::parse, which uses lpop. punk::lib installs the validated compat aliases
+                #(parity pinned by modules/punk/lib compat.test). No-op on Tcl 8.7+/9.
+                package require punk::lib
+            }
+        }
         testinterp eval {package require shellrun}
         #puts ">>>> [testinterp eval {
         #        set result ""