lib-management.dep-fetcher: probe engine.install_root + engine.config

Slice 6 of the dep-fetcher plan wired engine.install_root() and
engine.config(key) as Lua-level bindings. Use them when available;
keep the slice-5 env-var + hardcoded-URL fallbacks so this lib still
loads correctly under engines that pre-date the bindings.

Resolution chain is now:
  install_root : opts -> engine.install_root() -> SPOREL_INSTALL_ROOT
  gitea_base   : opts -> engine.config('gitea_base') -> hardcoded default

No version bump: behavior unchanged for callers that pass opts.* or
that run under engines without the new bindings.

init.lua

@@ -32,6 +32,49 @@
 
 local M = {}
 
+-- =====================================================================
+-- Engine binding probes (Slice 6)
+--
+-- Slice 6 of the dep-fetcher plan wired engine.install_root() and
+-- engine.config(key) as Lua-level bindings. We probe for them at
+-- call-time (not load-time) so this lib continues to load under
+-- engines that pre-date Slice 6 — the fallbacks (env var, hardcoded
+-- URL) keep slice-5-compat with unmodified hosts.
+-- =====================================================================
+
+local function resolve_install_root(opts)
+    -- 1. explicit override
+    if opts and opts.install_root then return opts.install_root end
+    -- 2. Slice 6 engine binding
+    if type(engine) == "table"
+       and type(engine.install_root) == "function" then
+        local ok, root = pcall(engine.install_root)
+        if ok and type(root) == "string" and root ~= "" then
+            return root
+        end
+    end
+    -- 3. Slice 5 stop-gap: env var
+    return os.getenv("SPOREL_INSTALL_ROOT")
+end
+
+local function resolve_gitea_base(opts)
+    -- 1. explicit override
+    if opts and opts.gitea_base then return opts.gitea_base end
+    -- 2. Slice 6 engine.config("gitea_base") (sourced from engine.json)
+    if type(engine) == "table"
+       and type(engine.config) == "function" then
+        local ok, val = pcall(engine.config, "gitea_base")
+        if ok and type(val) == "string" and val ~= "" then
+            return val
+        end
+    end
+    -- 3. Hardcoded slice-5 fallback
+    return "https://git.davoryn.de/sporel"
+end
+
+M._resolve_install_root = resolve_install_root  -- exposed for tests
+M._resolve_gitea_base   = resolve_gitea_base    -- exposed for tests
+
 -- =====================================================================
 -- Pure-Lua JSON decoder (strict subset: object / array / string /
 -- number / true / false / null). Sufficient for manifest.lib /
@@ -446,24 +489,28 @@ end
 -- ensure_for_module_at — full integrity check.
 --   module_manifest_path : absolute path to manifest.module
 --   opts:
---     install_root  : root of <staged>/libs/ — defaults to env
---                     SPOREL_INSTALL_ROOT (slice 6: engine.install_root()).
+--     install_root  : root of <staged>/libs/. Resolution chain (Slice 6):
+--                       1. opts.install_root (explicit override)
+--                       2. engine.install_root() if the binding is present
+--                       3. SPOREL_INSTALL_ROOT env var (slice-5 stop-gap)
 --     check_only    : skip per-lib git operations entirely
 --                     (used by closure-shape unit tests).
---     gitea_base    : override default base URL.
+--     gitea_base    : override default base URL. Resolution chain:
+--                       1. opts.gitea_base
+--                       2. engine.config("gitea_base") if available
+--                       3. hardcoded https://git.davoryn.de/sporel fallback
 function M.ensure_for_module_at(module_manifest_path, opts)
     opts = opts or {}
-    local install_root = opts.install_root
-        or os.getenv("SPOREL_INSTALL_ROOT")
+    local install_root = resolve_install_root(opts)
     if not install_root then
         return {
             ok = false, conflicts = {}, warnings = {},
             errors = {{
                 lib_id  = "<module>",
                 kind    = "config",
-                message = "no install_root: pass opts.install_root or " ..
-                          "set SPOREL_INSTALL_ROOT (slice 6 will add " ..
-                          "engine.install_root() binding)",
+                message = "no install_root: pass opts.install_root, " ..
+                          "set SPOREL_INSTALL_ROOT, or run under an engine " ..
+                          "that provides engine.install_root()",
             }},
             closure = {},
         }
@@ -507,8 +554,7 @@ function M.ensure_for_module_at(module_manifest_path, opts)
         end
     end
 
-    local gitea_base = opts.gitea_base
-        or "https://git.davoryn.de/sporel"
+    local gitea_base = resolve_gitea_base(opts)
 
     local warnings, errors = {}, {}
     run_state_check(unique, install_root, gitea_base, warnings, errors)
@@ -523,17 +569,20 @@ function M.ensure_for_module_at(module_manifest_path, opts)
 end
 
 -- Convenience wrapper resolving the module-id via install layout.
--- Slice 6 will replace SPOREL_INSTALL_ROOT with engine.install_root().
+-- Resolves install_root via the same Slice 6 chain as ensure_for_module_at:
+-- explicit opts → engine.install_root() → SPOREL_INSTALL_ROOT env var.
 function M.ensure_for_module(module_id, opts)
     opts = opts or {}
-    local install_root = opts.install_root
-        or os.getenv("SPOREL_INSTALL_ROOT")
+    local install_root = resolve_install_root(opts)
     if not install_root then
         return {
             ok = false, conflicts = {}, warnings = {},
             errors = {{
                 lib_id = "<module>", kind = "config",
-                message = "no install_root for ensure_for_module",
+                message = "no install_root for ensure_for_module: " ..
+                          "pass opts.install_root, set SPOREL_INSTALL_ROOT, " ..
+                          "or run under an engine providing " ..
+                          "engine.install_root()",
             }},
             closure = {},
         }