feat(stdlib): add Regex.* — a linear-time regular-expression engine (#18)

A regular-expression library with PCRE/PECL-compatible syntax, implemented as a
Thompson NFA / Pike VM so a bad pattern from a modder can NEVER cause
catastrophic backtracking — matching is O(n·m), never exponential. `(a+)+$` on
40 non-matching chars, `(a*)*b`, `(.*a){20}b` all run in microseconds; a 50 KB
input scans in ~7 ms.

The engine (runtime/native/regex.ludic + regex_vm.ludic, ~700 lines of Ludic, no
C) parses a pattern to a small bytecode program — an unanchored lazy `.*?` prefix
makes a plain search match anywhere — and the VM runs every alive thread in
lockstep per input byte, deduped by program counter and carrying capture slots
(save/restore, leftmost-greedy priority). Supported: literals, `.`, classes
`[...]` (ranges, negation, `\d \w \s` and their negations), anchors `^ $`,
alternation `|`, capturing and `(?:…)` groups, and `* + ? {n} {n,} {n,m}` in
greedy or lazy form, plus the common escapes; numbered capture groups. Errors are
values — an invalid pattern compiles to null, never a crash. Backreferences and
look-around are out of scope for a linear engine, and on the degenerate case of a
nullable subpattern under an unbounded quantifier positions may differ from a
backtracking engine (the price of the linear-time guarantee) — documented.

Surface (Regex.*, aliased in emit_call.ludic to the regex_* functions):
compile / valid / matches / test / find / exec / next / replace / group /
group_count / start / end / ok.

The runtime is spliced on demand: the parser sets a flag when it sees `Regex.`
and maybe_splice_runtime imports the engine — so it costs nothing in a program
that doesn't use it and works in a plain tool (not just an ECS game).

Verified against Python's `re` as an oracle: a 20k-case grammar fuzzer agrees
100% on realistic patterns (0 / 15000 with capture groups) and 99.8% on group-0
spans across the full pathological grammar, the residual being the documented
nullable-quantifier case. examples/library/regex.ludic asserts the behaviour
(wired into `x test`, now 60 passed); docs: a Regex section + 13 per-symbol
pages, inventory + coverage green. Seed reseeded; the C-free bootstrap fixpoint
holds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/backend/emit_call.ludic

@@ -185,6 +185,24 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
     if (meth == "write") { bare = "save" }
     if (meth == "read")  { bare = "load" }
   }
+  # Regex.* -> the regex_* engine functions (spliced from runtime/native/regex*.ludic
+  # when a program mentions Regex.*). Each is a plain alias; the engine functions
+  # are ordinary Ludic, so the generic call path resolves them to @fn_regex_*.
+  if (ns == "Regex") {
+    if (meth == "compile")     { bare = "regex_compile";     push(labels, "pattern") }
+    if (meth == "valid")       { bare = "regex_valid";       push(labels, "pattern") }
+    if (meth == "matches")     { bare = "regex_matches";     push(labels, "text"); push(labels, "pattern") }
+    if (meth == "test")        { bare = "regex_test";        push(labels, "text"); push(labels, "re") }
+    if (meth == "find")        { bare = "regex_find";        push(labels, "text"); push(labels, "pattern") }
+    if (meth == "exec")        { bare = "regex_exec";        push(labels, "text"); push(labels, "re") }
+    if (meth == "next")        { bare = "regex_next";        push(labels, "text"); push(labels, "re"); push(labels, "from") }
+    if (meth == "replace")     { bare = "regex_replace";     push(labels, "text"); push(labels, "pattern"); push(labels, "replacement") }
+    if (meth == "group")       { bare = "regex_group";       push(labels, "match"); push(labels, "n") }
+    if (meth == "group_count") { bare = "regex_group_count"; push(labels, "match") }
+    if (meth == "start")       { bare = "regex_start";       push(labels, "match"); push(labels, "n") }
+    if (meth == "end")         { bare = "regex_end";         push(labels, "match"); push(labels, "n") }
+    if (meth == "ok")          { bare = "regex_ok";          push(labels, "match") }
+  }
   if (bare == null) { perr(`unknown builtin {ns}.{meth}`) }
   reorder_named(e, labels)
   let id = node(E_ID); id.s = bare; e.a = id