Rework diagnostic logging: fix the file writer, add rotation and a ring buffer

Implements steps 1-3 of discussion #644 (deliberately not steps 4/5 --
no signal handler / crash flush, no diagnostics UI; see below).

## Step 1 -- fix the existing logger (bugs, no new behavior)

- One QFile handle held open for the whole session under a mutex,
  instead of opening and closing the log file on every single message.
- The log directory and the session's date-stamped filename are
  resolved exactly once, in the new QetLogger::init() called explicitly
  from main() immediately before qInstallMessageHandler() -- not
  recomputed per message, so a session that runs past midnight now
  stays in one file instead of silently splitting.
- Age-based retention now uses lastModified() instead of lastRead():
  opening a log to attach it to a bug report no longer resets its
  retention clock.
- stderr and file output both encode UTF-8 explicitly (toUtf8()),
  replacing stderr's toLocal8Bit() and the file stream's previously
  Qt5/Qt6-inconsistent default encoding.

## Step 2 -- size-capped rotation + hardening

- The previously-unbounded daily file is now capped at 2 MiB and
  rotated (kMaxFileBytes/kRotationKeep in QetLogger), keeping
  <date>.log plus <date>.1.log .. <date>.4.log; oldest is dropped.
- Each message is truncated to 4 KB with a "...[truncated N bytes]"
  marker before it reaches the ring or the file.
- Control characters (newlines, tabs, other non-printables) in message
  content are escaped, since much of what QET logs is externally
  controlled (file paths, element names, font strings out of a .qet
  file) -- left unescaped, an embedded '\n' could forge log lines.
- The log file is refused if a symlink already exists at that path,
  and is created/rotated owner-read/write only.

## Step 3 -- in-memory ring buffer

- LogRing (sources/logging/logring.h) is a fixed-capacity, always-on
  ring of the last 4096 log lines, preallocated once at construction
  (4096 * 512 B = 2 MiB) so append() never allocates. Entries are
  stored as plain pre-formatted bytes in fixed-size slots -- the shape
  discussion #644 specifies so a *future* crash handler could dump it
  with nothing but write(2), even though no such handler exists yet.
  Thread-safe via a plain QMutex (the lock-free requirement in the
  discussion applies specifically to a signal-handler read path, which
  this step doesn't add).

## Escape hatch

QET_LOG_DISABLE=1 in the environment at startup bypasses all of the
above -- no ring, no file, no rotation -- falling back to a minimal,
self-contained stderr passthrough that doesn't share any code with the
new formatting/sanitization path, so it stays usable even if that path
is what's misbehaving.

## Deliberately not included (per the discussion's own phasing)

- No signal handler / crash-time ring flush (step 4) -- the discussion
  flags this as the highest-risk piece, explicitly meant to land last
  and behind its own switch once the rest is proven.
- No diagnostics export UI (step 5).
- No log categories, session header, repeat collapsing or rate
  limiting -- listed under "best practices worth building in", not
  part of steps 1-3.

## Testing

Built clean, no new warnings.

Verified with real runs (QT_QPA_PLATFORM=offscreen, isolated HOME):
- Log file created at the expected dataDir()/YYYYMMDD.log path, mode
  0600.
- A full startup's worth of real messages (translations, MachineInfo's
  system dump, collection loading) written correctly; every one of the
  231 lines in one run starts with a proper timestamp -- confirmed the
  sanitizer correctly escapes the raw embedded newlines/tabs in
  MachineInfo's multi-line CPU/GPU description fields into visible
  \n/\t sequences rather than letting them fragment the log.
- QET_LOG_DISABLE=1: zero log files created, stderr still worked via
  the independent legacy path.
- Rotation: pre-filled a log to just under the 2 MiB cap, ran a normal
  session, confirmed it rotated to <date>.1.log (still 0600) with a
  byte-clean split (no truncated/duplicated line at the boundary) and
  a fresh <date>.log picked up from the next line.

sources/logging/logring.h

@@ -0,0 +1,77 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef LOGRING_H
+#define LOGRING_H
+
+#include <QByteArray>
+#include <QMutex>
+#include <QVector>
+#include <vector>
+
+/**
+	@brief The LogRing class
+	Fixed-capacity, always-on in-memory ring of the most recent log
+	lines. Discussion #644 (step 3): the ring exists as forward-compatible
+	infrastructure for a future crash-flush (step 4, not implemented
+	here) as well as an on-demand "what just happened" snapshot, so its
+	entries are stored pre-formatted as plain bytes in storage
+	preallocated once at construction -- append() never allocates.
+
+	Entries are fixed-size slots rather than a byte-packed ring: with
+	kCapacityEntries * kEntryBytes chosen to land exactly on the 2 MiB
+	budget, this keeps wraparound trivial (whole-slot overwrite, so a
+	slot is always either fully the old entry or fully the new one --
+	no torn entries) at the cost of truncating any single line to
+	kEntryBytes, independently of the logger's own (larger) per-message
+	truncation.
+
+	Thread-safe via a plain QMutex. This is *not* the lock-free design
+	discussion #644 specifies for a signal-handler crash path (step 4)
+	-- no signal handler is installed by this code, so nothing calls
+	into the ring from inside a signal context.
+*/
+class LogRing
+{
+	public:
+		static constexpr int kCapacityEntries = 4096;
+		static constexpr int kEntryBytes = 512; // 4096 * 512 = 2 MiB total
+
+		LogRing();
+
+		/// Append one already-formatted, already-truncated log line.
+		/// Bytes beyond kEntryBytes - 1 are dropped with a truncation marker.
+		void append(const QByteArray &line);
+
+		/// Snapshot of the entries currently held, oldest first.
+		QVector<QByteArray> snapshot() const;
+
+		void clear();
+
+	private:
+		struct Entry {
+			char data[kEntryBytes] = {};
+			int length = 0;
+		};
+
+		mutable QMutex m_mutex;
+		std::vector<Entry> m_entries; // preallocated once, capacity fixed
+		int m_next_index = 0;
+		int m_count = 0;
+};
+
+#endif // LOGRING_H