HUB75: use FM6124 driver for 4-scan panels, remove 64x64 limitation if board has PSRAM (partial solution for #5628)

[softhack007]

• some 4-scan "outdoor" panels need the FM6124 driver. Tests on my hardware showed that "shiftreg" panels seem to work with this driver, so the risk of regressions is low
• partial solution for HUB75 not supporting FM6124 and FM6126A chips #5628
• Remove 1-panel limit if the board has PSRAM.
• Remove an unnecessary mxconfig.double_buff == false check (we never use the driver's internal double buffer)

Summary by CodeRabbit

Bug Fixes

• HUB75 LED matrix driver now validates hardware compatibility more thoroughly and automatically adjusts configuration settings when needed for improved reliability.

[coderabbitai]

Walkthrough

BusHub75Matrix initialization adds PSRAM validation to enforce single-panel mode on PSRAM-constrained boards, explicitly configures FM6124 as the I2S shift-register driver for TYPE_HUB75MATRIX_QS, documents available driver options, and removes the conditional guard around _ledBuffer allocation to allocate it unconditionally.

Changes

HUB75 Matrix Initialization

Layer / File(s)	Summary
PSRAM validation and FM6124 driver configuration wled00/bus_manager.cpp	Added PSRAM presence and size validation that enforces single-panel mode when PSRAM is unavailable on 64-pixel boards; explicitly configures HUB75 I2S driver to FM6124 for TYPE_HUB75MATRIX_QS; added comment enumerating additional shift-register driver options.
Unconditional LED buffer allocation wled00/bus_manager.cpp	Removes the conditional check on mxconfig.double_buff that previously guarded _ledBuffer allocation, allowing unconditional buffer allocation at initialization time.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related issues

• wled/WLED#5628: Adds explicit FM6124 driver configuration for TYPE_HUB75MATRIX_QS, directly addressing the missing HUB75 I2S driver initialization.

Possibly related PRs

• wled/WLED#4950: Modifies wled00/bus_manager.cpp HUB75/HUB75MATRIX_QS initialization around panel chaining and chain_length handling.
• wled/WLED#5026: Modifies wled00/bus_manager.cpp BusHub75Matrix initialization for HUB75 chain-length handling and QS-type behavior/buffering.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main changes: using FM6124 driver for 4-scan panels and removing the 64x64 limitation when PSRAM is available, directly matching the implementation changes in bus_manager.cpp.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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