lib/regex: fold-2b tranche A — thread/newmatch types + leaf fns

thread (regex.ha:55-64) and newmatch (ha:66) land verbatim ahead of
their engine consumers; result_free (ha:1113-1116) and strerror
(ha:1126-1127) complete the exported error/result surface.

delete_thread/add_thread deferred behind #15 (append-through-ptr)
and #17 (deref-spine element reads); is_consuming_inst deferred
behind #19 (>48B tagged by-value call boundary unwired + divergent
callee receive) — noted at the Hare-order site.

Tests are row-table driven: thread_shape reads back both appended
threads against a [2]texp want table (root_capture rows deferred —
every read route is compiler-blocked, #6/#7, probed at HEAD);
newmatch_discriminates drives one row per (void|newmatch|nomem)
member, incl. a nomem-vs-newmatch row; result_free also covers the
zero-header empty result (find()'s ha:915-916 no-match shape).
Byte-id re-verified on the regenerated combined: the pre-existing
FC0 regex.finish hunk is the only divergence.

test/wcc/989_regex_run.c

@@ -1,6 +1,7 @@
 /*
- * 989_regex_run — execute the lib/regex fold-1 @test fixture under the
- * C-side `ww run` driver and assert exit 0.
+ * 989_regex_run — execute the lib/regex @test fixture (fold-1 data
+ * model + fold-2a compile() + fold-2b tranche-A scaffolding) under
+ * the C-side `ww run` driver and assert exit 0.
  *
  * Sibling to 984_base64_run / 989_sha256_run (9xx is full so this
  * shares the 989 prefix — the `short` name keys the binary, cf the