fix(boot): keep QR scannable on landscape 800x600+ boot screens

[davelee98]

Problem

On the reTerminal E1004 (1200x1600 dual-controller Spectra6) rotated to 1600x1200, the boot screen's middle zone had huge text and a QR code squeezed down to 2–3 px modules in the corner. This isn't specific to dual-controller panels. The same thing happened on every landscape panel ≥ 800x600, including 1304x984, 960x640 and 800x600. Only 1872x1404 avoided it, because of its hard-coded ultraHiResPanel scale offset.

Root cause: the layout search in writeBootScreenWithQr() tried text scales from the largest down. It accepted the first scale at which any QR module size fit, even 1 px. So the text always stayed at its largest size and the QR took whatever space was left.

Fix (src/boot_screen.cpp)

• New bootQrModuleMin(): returns half of bootQrModuleMax() on panels whose cap is ≥ 8 (≥ 800x600), and 1 on smaller panels.
• New moduleMin parameter in bootLayoutFit(): the module-size loop stops at moduleMin instead of going down to 1.
• Two-pass search: pass 0 enforces the minimum module size and shrinks the text until the layout fits. Pass 1 is the original any-size search. Panels below 800x600 start at pass 1, so they behave exactly as before.
• Follow-up commit: renames the loop variable to fitPass (so it doesn't shadow round()) and corrects the comment. No behaviour change.

Performance: the search runs once at boot and does at most about 10 × 16 iterations of integer math. There is no heap allocation, no new globals and no change to the row-streaming path.

Verification

• Host render harness: compiles the real boot_screen.cpp against stubs and renders 14 panel sizes, including the E1004 in both orientations with the split stream.
  • The E1004 at 1600x1200 goes from text scale 10 / 3 px modules to text scale 8 / 9 px modules.
  • Portrait E1004, 1872x1404, 1024x576, 800x480, 640x384, 600x448, 400x300, 480x800 and 296x128 are byte-identical to before.
• Adversarial review: a sweep of 2,115 logical sizes (400–2648 px each way) found no overlap, clipping, out-of-bounds QR or loss of the QR's blank margin on any size this changes.
  • Every size that changed got a bigger QR and text that is 1–2 scale steps smaller.
  • A few near-square landscape sizes (e.g. 1700x1600, 1232x1028) switch from the stacked layout to text-left/QR-right, and render better that way.
• Firmware build: all 12 CI targets build green on the branch head (54cf579). The change still needs a look on a real E1004.

Known, not addressed here

The existing qrX = contentRightX - modulePx*(qrSize+quiet) override leaves the QR's right blank margin narrower than the standard 4 modules (about 2.7–3.6) on some unchanged sizes (1872x1404, 2560x1440, 1024x576, 800x480). A possible follow-up fix is to clamp qrX to w_log - qrPx.