Perf/UI allocation reduction

[robekl]

Summary

This branch reduces avoidable UI-time heap allocation in two hot paths:

• Reuses a persistent scanline buffer in the ST7789 flush path instead of allocating/freeing a temporary pixel buffer on each row update.
• Rewrites OLEDDisplay::drawString() to scan the source string directly instead of copying it with strdup() and splitting it with strtok().

Changes

ST7789 scanline buffer reuse

What changed:

• Added a persistent _pixbuf scratch buffer and _pixbuf_capacity to ST7789Spi.
• Allocates the scratch buffer in connect() when needed and reuses it across display() calls.
• Removes per-iteration rtos_malloc() / rtos_free() calls from both ST7789 display() code paths.
• Frees the scratch buffer in the destructor.
• Preserves the existing buffer if a resize/allocation attempt fails.

Why:

• display() was doing repeated heap allocation in a render hot path.
• Reusing a single buffer reduces allocator churn and fragmentation risk on embedded targets.

OLED drawString heap removal

What changed:

• Replaced strdup() + strtok() in OLEDDisplay::drawString() with direct pointer-based scanning over strUser.c_str().
• Keeps vertical centering logic intact.
• Preserves the previous behavior of skipping empty newline-only segments.

Why:

• drawString() previously allocated and copied the full input string on every call.
• Rendering now avoids heap allocation entirely for this path.

Behavior

Expected behavior is unchanged aside from allocation strategy:

• ST7789 rendering still produces the same converted scanline data.
• OLEDDisplay::drawString() still renders the same logical lines and alignment behavior as before.

Benefit

These changes remove unnecessary heap allocation from two UI rendering paths.

• Lower runtime allocation churn during display updates.
• Reduced risk of heap fragmentation on long-running embedded devices.
• More predictable rendering performance, especially on frequent redraw paths.
• Same behavior with less transient memory traffic.