mdview2: горизонтальный скролл по типу блока + фиксы fence/HR/SDCC

Горизонтальный скролл (Фаза 5, финал): - Скроллим по ТИПУ, а не по длине: новый флаг IF_HSCROLL ставится только на код и таблицы; HR и границы fence (IF_NOWRAP без IF_HSCROLL) не двигаются. Блок едет целиком, включая строки короче 80. - Обход бага кодогенерации SDCC z80: `if (n!=g) g=n;` пишет (n-g) вместо n (SUB сравнения затирает A, store переиспользует испорченный A). Лечится записью viewport_x ДО сравнения. Минимальный репродьюсер и оба описания для трекера — в docs/bugs/sdcc-z80-cmp-store-a/ (воспроизводится на чистом sdcc 4.5, в т.ч. с --no-peep → это кодогенератор, не peephole). Рендеринг: - Отступленный fence (```c внутри списка) теперь распознаётся: is_fence_raw пропускает ведущие пробелы/табы; то же в рендере прячет строку-границу. - Строки-разделители (HR, ровно 80) больше не участвуют в скролле. Чистка: удалён мёртвый код (is_fence_delim, get_init_style[_raw], is_cont, seg_flags). Makefile (mdview/mdview2): iconv UTF-8→CP866 завершается ненулевым кодом при отбрасывании символов — игнорируем (|| true). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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+# [z80] `global = local` after `if (local != global)` stores `local - global` (A clobbered by comparison)
+
+## Summary
+
+On the z80 backend, the sequence
+
+```c
+if (n != g) { g = n; }
+```
+
+where `n` is in register `A` and `g` is a global, miscompiles. The compiler
+evaluates the `!=` comparison with `SUB A,(HL)`, which destroys `A`, and then
+emits the assignment `g = n` by storing `A` — which now holds `n - g`, not `n`.
+As a result the global ends up holding `(unsigned char)(n - old_g)` instead of `n`.
+
+It should either use `CP (HL)` (which leaves `A` intact) for the comparison, or
+reload `n` before the store.
+
+## Version
+
+SDCC 4.5.0 #15242 (Mac OS X x86_64). Default options; also reproduces with
+`--opt-code-speed` and with `--no-peep` (so this is a code-generator bug, not a
+peephole-optimizer bug).
+
+## Minimal reproducer
+
+```c
+unsigned char vx;
+
+void update(unsigned char n)
+{
+    if (n != vx) {
+        vx = n;
+    }
+}
+```
+
+Build:
+
+```
+sdcc -mz80 -S repro.c
+```
+
+## Generated assembly (wrong)
+
+```asm
+_update::
+;repro.c: if (n != vx) {
+	ld	hl, #_vx
+	sub	a, (hl)        ; A (= n) is destroyed: A = n - vx
+	ret	Z
+;repro.c: vx = n;
+	ld	(_vx+0), a     ; stores (n - vx) instead of n
+;repro.c: }
+	ret
+```
+
+With `--no-peep` the same defect is present (only the branch shape differs):
+
+```asm
+_update::
+	ld	iy, #_vx
+	sub	a, 0 (iy)      ; A (= n) destroyed
+	jp	NZ, 00112$
+	jp	00103$
+00112$:
+	ld	(_vx+0), a     ; stores (n - vx)
+00103$:
+	ret
+```
+
+## Why it happens
+
+`n` arrives in `A` (sdcccall). The code generator picks `SUB A,(HL)` to evaluate
+the relational `n != vx`. `SUB` overwrites `A` with the difference. The generator
+then treats the still-live value `n` as if it were still in `A` and emits a bare
+store `LD (_vx),A` for the assignment, without reloading `n` first. Because the
+defect survives `--no-peep`, it is in code generation (register/lifetime tracking
+across the comparison), not in the peephole optimizer.
+
+The correct lowering for the comparison is `CP (HL)`, which sets the flags exactly
+like `SUB` but preserves `A`, so the subsequent store would be correct with no
+extra instructions.
+
+## Variants that also reproduce
+
+- `if (n == vx) return; vx = n;` (early-return form)
+- `n` coming from a function call result instead of a parameter
+- both `-mz80` default and `--opt-code-speed`
+
+## Workaround
+
+Store into the global *before* the comparison, so the destructive `SUB` is no
+longer on the store path; compare a saved copy instead:
+
+```c
+void update(unsigned char n)
+{
+    unsigned char old = vx;
+    vx = n;                 /* store first, A still holds n */
+    if (n != old) {
+        /* side effect */
+    }
+}
+```
+
+generates the correct:
+
+```asm
+_update::
+	ld	(_vx+0), a
+	ret
+```
+
+## Files in this directory
+
+- `repro.c` — minimal reproducer
+- `repro.asm` — generated output, default options (defect visible)
+- `repro.nopeep.asm` — generated output with `--no-peep` (defect still present)
+- `workaround.c` / `workaround.asm` — store-before-compare workaround (correct)
+
+## Tracker search
+
+A search of the SDCC bug tracker did not turn up an exact duplicate. The closest
+version-matching report, #3834 "[Z80][SDCC 4.5] Compiler bug", is a *different*
+defect (`genPointerSet`, swapped push/pop order), not this comparison-clobbers-A
+case.