Shell: Tab completes a path on the SD card (#67)

Past the command's name, Tab completes the word being typed as a path: a
folder keeps its slash to go on from, a file completed whole gets a space,
several candidates are listed. Any case typed, the name's own is taken.
After a file command the first slash is understood (`cat no` is /no).
The folder is read on the storage task, bounded to 400 entries looked at
and 24 candidates.

489 host tests (2 new). Checked on the device: a folder, a file inside it,
several candidates, no slash, another case, nothing matching.

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

docs/milestones/S1.md

@@ -149,7 +149,7 @@ The console's commands could only be typed on a PC: over USB, or over Wi-Fi with
 | Q205 | **Trusted like USB serial**, not like the network: `debug on` and `debug token` work from it, as they do in Settings. The token is never shown. |
 | Q206 | *Revised the same day, after trying it.* **It shows the replies to its own commands, and only those.** The console knows who each line is printed for (`Console::Origin`): a command run from the Shell prints as the Shell's, and so does what answers it later from another task, which notes who asked and takes it back when it prints (`ls`, `tasks`, `du`, `cp`, `update check`, `sd list`, `screenshot`, `gemini get`). What USB or the Debug Console asked for, and the system's own lines, are not the Shell's. **Ctrl+b shows everything** instead. The first version kept whatever was printed in the ten seconds after a command, which was a guess, and a noisy one. |
 | Q207 | **Nothing while it's closed.** Open, a 4 KB ring of the console's and up to 4 KB of lines; both go when the App is left, with the list of commands for Tab. |
-| Q208 | Enter runs the line; Fn with up and down recalls the last 16; Alt with up and down scrolls back; **Tab completes the command's first word** from the firmware's `help` text. Paths aren't completed. |
+| Q208 | Enter runs the line; Fn with up and down recalls the last 16; Alt with up and down scrolls back. **Tab completes** the command's first word, from the firmware's `help` text, and *(added the same day)* **past it a path on the SD card**: a folder keeps its slash to go on from, a file completed whole gets a space, several candidates are listed. Whatever the case typed, the name's own is taken, since the card doesn't tell them apart. After a file command the first slash is understood (`cat no` is `/no`). A name with a space in it isn't completed. |
 | Q209 | *Revised the same day.* **`rm` is Unix's, with a question.** A folder needs `-r`, here and over the consoles (where `rm <folder>` used to remove it with what was in it). In the Shell, a file, or a folder with something in it, is asked about unless `-f` (`-rf`, `-r -f`); an empty folder with `-r` goes without a word; what `rm` would refuse anyway, it refuses itself. Over the consoles nothing is asked: scripts delete as before. **`screenshot [seconds]`** saves the screen as a PNG in `/screenshots`, now or after a pause, since from the Shell "now" is the Shell. `get`, `put`, `coredump get` and `reset` answer `Debug Console only`. |
 | Q210 | Commands are echoed as `> command` into the console, so a session reads the same from afar. **A token being set is not echoed.** |
 | Q211 | **Not in Safe Mode**, which starts no Apps: issue #77. |
@@ -164,7 +164,8 @@ The console's commands could only be typed on a PC: over USB, or over Wi-Fi with
 - **`info` says which App is in front** (`app: Shell`): so that a hand driving the device from afar can look before it types.
 - **`screenshot`** writes the PNG a row at a time with no buffer: 8-bit indexed colour, the 256 colours of RGB332 as the palette, the pixels in one stored deflate block (`lib/files/src/png_rgb332.h`, 4 tests). 33,383 bytes for the 240 x 135 screen. A Toast says so once it is on the card, so the Toast is never in the picture.
 - **The `help` text lost its shorthand** (`update check | list | status` is written out), since Tab reads its first words from there.
-- **Cost:** 16 KB of flash, 56 bytes of static RAM. Open: 7 KB of heap (107.6 KB free before, 100.7 with it open, 106.1 after leaving).
+- **Tab on a path** reads the folder on the storage task while the main loop waits, so it is bounded: 400 entries looked at, 24 candidates given back, and `...` after the list when there were more.
+- **Cost:** 21 KB of flash, 96 bytes of static RAM. Open: 7 KB of heap (107.6 KB free before, 100.7 with it open, 106.1 after leaving).
 
 ### What went wrong while building it
 
@@ -188,6 +189,7 @@ Two changes came of it. The Shell now **queues** its line and the main loop runs
 | `rm -r` on an empty folder | Removed, no question |
 | `rm -r` on a folder with files | Asks; Cancel leaves it. `rm -rf` removes it without asking |
 | `rm` on a file | Asks; Delete removes it |
+| Tab on a path | `ls /no` becomes `ls /notes/`, and again, with one note in it, the note's whole name and a space. `ls /g` lists `gemini/  gnss/`. `cat no` becomes `cat /notes/`. `rm -r /CAP` becomes `rm -r /captures/`. `ls /zz` stays as it is |
 | `No`, Tab, Enter | Completes to `Notes ` and opens Notes. `Shell` from the Debug Console opens the Shell |
 | `screenshot 4`, then Home | The picture, taken four seconds later, is of the Launcher: the pause works, and the Toast isn't in it |
 | `quit` | Back to the Launcher, and the memory comes back |