F1 step 1: names, read-only rules and the packed listing (host-tested)

lib/files: path parts; names checked for rename and new folder; why
something can't be renamed, moved or deleted (the Gemini cache, a file
being written or a folder holding one, the firmware's top-level
folders), and why it can't go into a folder; the viewer for a file by
its name, with a sniff for text. FileList keeps a folder's entries
packed, 256 at most, the first 256 by name whatever order the card lists
them in, and sorts by name, date or size with folders first.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

lib/files/src/file_names.h

@@ -0,0 +1,42 @@
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+#include <string>
+#include <vector>
+
+namespace roro::files {
+
+// Paths on the SD card are absolute and use '/': "/gemini/saved/index.gmi".
+std::string parentOf(const std::string& path);    // "/" for a top-level entry and for "/"
+std::string baseName(const std::string& path);    // "" for "/"
+std::string joinPath(const std::string& folder, const std::string& name);
+std::string extensionOf(const std::string& name);  // lower case, without the dot; "" if none
+bool isInside(const std::string& path, const std::string& folder);  // a folder is inside itself
+
+// A name typed for rename or a new folder (F1, Q131): "" if FAT and this App can take it, or why not.
+std::string checkName(const std::string& name);
+
+// The top-level folders the firmware keeps its files in. They can't be renamed or deleted;
+// what's in them can (Q130).
+extern const char* const kFirmwareFolders[];
+extern const size_t kFirmwareFolderCount;
+constexpr const char* kGeminiCache = "/gemini/cache";
+
+// Why `path` can't be renamed, moved or deleted, or "" if it can (Q130). `inUse`: the files the
+// firmware has open right now.
+std::string whyReadOnly(const std::string& path, const std::vector<std::string>& inUse);
+
+// Why `source` can't be copied or moved into the folder `into`, or "" if it can.
+std::string whyNotInto(const std::string& source, const std::string& into);
+
+// "a (2).gmi": the name of a copy made next to its original.
+std::string copyName(const std::string& name, int n);
+
+// Which viewer opens a file (Q134), from its name. Unknown: look at the first bytes.
+enum class FileKind : uint8_t { Text, Gpx, Pcap, Ota, Unknown };
+FileKind kindOf(const std::string& name);
+bool opensAtEnd(const std::string& name);  // logs
+bool looksLikeText(const uint8_t* data, size_t len);
+
+}  // namespace roro::files