feat(pkg): package manager — fetch + MVS resolve + namespace registration (#63)

Implements the v1 direction decided in the RFC as a set of `x` subcommands
plus a small, contained compiler change.

  * URL-as-identity, no registry — a dependency is named by its git import
    path and a `git tag vX.Y.Z` publishes a version.
  * Minimum Version Selection — a `require` is a minimum; the resolver picks
    the greatest required minimum per module, then the reachable closure at
    those versions. Deterministic, no SAT solver (tools/x/pkg.ludic).
  * Content-addressed global store + per-project links — packages live once in
    ~/.ludic/store keyed by a content hash; each project links them under
    ludic_modules/. package.ludic (manifest) + package.lock.ludic (lock).
  * Namespace registration for source packages via a module-root import
    fallback in the compiler: do_import resolves a non-local, non-absolute
    import under $LUDIC_MODULES (default ludic_modules/), so a fetched
    package's Ludic compiles into the consumer the way the built-in stdlib
    does. Collisions and missing prebuilt targets are hard errors.

Commands: x add / x get / x update / x verify / x vendor. New hermetic suite
`x test-pkg` (stands up throwaway git repos, offline) is gated inside `x test`.

Existing programs compile byte-for-byte identically (the import fallback only
fires when the local path is absent); the C-free bootstrap fixpoint holds and
the seed is regenerated. Full suite: 87 passed, package suite: 12 passed.

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

selfhost/frontend/parse.ludic

@@ -573,12 +573,26 @@ function parse_test() -> Node {
   return n
 }
 
-# lex and parse an imported fragment into `prog`, saving/restoring lexer state
+# lex and parse an imported fragment into `prog`, saving/restoring lexer state.
+#
+# An import is resolved first relative to the importing file (the historical
+# behaviour). When that fails and the spec is not absolute, it is looked up
+# under the package module root — $LUDIC_MODULES, default "ludic_modules" — so a
+# fetched source package resolves by its import path, e.g.
+#   import "git.workshopsoft.io/user/pkg/foo.ludic"
+# materialised by `x get` at ludic_modules/git.workshopsoft.io/user/pkg/foo.ludic.
+# The fallback only fires when the local path does not exist, so every existing
+# import resolves exactly as before (the emitted IR is byte-identical).
 function do_import(rel: pointer) -> void {
-  let full = path_join(cur_dir, rel)
+  var full = path_join(cur_dir, rel)
+  var src = read_file(full)
+  if (src == null) and (rel[0] != 47) {
+    let cand = ensure_slash(getenv_or("LUDIC_MODULES", "ludic_modules")) + rel
+    let s2 = read_file(cand)
+    if (s2 != null) { full = cand;  src = s2 }
+  }
   if already_loaded(full) { return }
   push(loaded_paths, full)
-  let src = read_file(full)
   if (src == null) { perr(`cannot open import {full}`) }
   let saved_toks = toks;  let saved_pi = pi;  let saved_dir = cur_dir
   cur_dir = dir_of(full)