feat(launcher): add frontmatter.lua JSON-frontmatter parser

Parses manifest.launcher.md leading JSON-frontmatter (delimited by
standalone '---' lines), returns (meta, body). Reuses
lib-management.dep-fetcher._json_decode so we do not add a second JSON
decoder to the codebase. Lazy require so test-lib contexts can swap a
stub decoder via _G.LAUNCHER_FRONTMATTER_DECODER.

frontmatter.lua

@@ -0,0 +1,63 @@
+-- frontmatter.lua — JSON-Frontmatter parser for manifest.launcher.md
+-- Splits on the first standalone '---' line. Everything before goes to
+-- depf._json_decode; everything after (after a single LF) is body.
+-- Loud-Error semantics: parse failure returns nil + err-string.
+
+local M = {}
+
+-- depf is required lazily so this file can be dofile'd into a test-lib
+-- context where lib-management.dep-fetcher is not on the dep list. The
+-- launcher main module does require it eagerly; the test-lib monkeys
+-- a stub decoder into _G before calling fm.parse.
+local function get_decoder()
+    if _G.LAUNCHER_FRONTMATTER_DECODER then
+        return _G.LAUNCHER_FRONTMATTER_DECODER
+    end
+    local ok, depf = pcall(require, "lib-management.dep-fetcher")
+    if ok and depf and depf._json_decode then return depf._json_decode end
+    return nil
+end
+
+-- Splits the content at the first line equal to '---'. The first such
+-- line ends the JSON region; lines before it are JSON, lines after
+-- are body. Both regions are LF-joined back into strings.
+local function split_at_separator(content)
+    local lines = {}
+    for line in (content .. "\n"):gmatch("([^\n]*)\n") do
+        lines[#lines + 1] = line
+    end
+    -- A valid file starts with '---' (no leading whitespace).
+    if lines[1] ~= "---" then return nil end
+    local sep_idx = nil
+    for i = 2, #lines do
+        if lines[i] == "---" then sep_idx = i; break end
+    end
+    if not sep_idx then return nil end
+    local json_lines = {}
+    for i = 2, sep_idx - 1 do json_lines[#json_lines + 1] = lines[i] end
+    local body_lines = {}
+    for i = sep_idx + 1, #lines do body_lines[#body_lines + 1] = lines[i] end
+    return table.concat(json_lines, "\n"), table.concat(body_lines, "\n")
+end
+
+function M.parse(content)
+    if type(content) ~= "string" then
+        return nil, nil, "frontmatter.parse: content must be string"
+    end
+    local json_str, body = split_at_separator(content)
+    if not json_str then
+        -- No frontmatter — treat the whole content as body, empty meta.
+        return {}, content
+    end
+    local decode = get_decoder()
+    if not decode then
+        return nil, nil, "frontmatter.parse: no JSON decoder available"
+    end
+    local meta, derr = decode(json_str)
+    if not meta then
+        return nil, nil, "frontmatter.parse: " .. tostring(derr)
+    end
+    return meta, body
+end
+
+return M