Fix ghostel-debug-info report truncation and dead key-encoding probe

[dakra]

Problem

M-x ghostel-debug-info produced a nearly empty report: only a headerless tail of the key-encoding table plus the non-default-settings section survived, and the encoder chords all showed (no output).

Two regressions from the 0.40 renderer/PTY rework:

1. Report wiped by the probe terminal. Since the renderer became buffer-affine (679b113), ghostel--new erases the current buffer on init. The key-encoding probe was created with *ghostel-debug* current, silently wiping every section inserted before it (System, Ghostel, Frame, Environment, Buffer, Process, …). No error was signaled.
2. Encoder byte capture dead. The probe captured bytes by overriding the elisp ghostel--write-pty, but the native PTY path (40f1269) writes the PTY directly from Zig and never calls it — and silently drops bytes when no process is live. Only the elisp raw-fallback chords (M-f/M-b/M-.) still showed bytes. ghostel-debug-keypress had the same blindness via its ghostel--write-pty advice.

Fix

• Create the probe terminal and run all encodes inside with-temp-buffer, so the buffer-affine init can't touch *ghostel-debug*.
• ghostel--encode-key now returns the encoded bytes as a unibyte string (nil when the encoder produced nothing) instead of t. The live send path only tests truthiness and is unchanged; no extra heap on the Zig side (the stack buffer moved to the caller). The probe reads bytes from the return value, and ghostel-debug-keypress records them via :filter-return advice.

Verification

• make -j8 all passes; two new native ERT tests (report survives the probe; ghostel--encode-key returns 0x7f for Backspace).
• Live-verified under elate: all 13 report sections present, every chord shows real bytes (Backspace → 0x7f, C-M-f → 0x1b 0x06, M-Backspace → 0x1b 0x7f), and ghostel-debug-keypress on Backspace records encode-key: hex 7f.