chore(spiflash): improve the efficiency of "is block erased" detection

[richardclli]

The spi_flash.cpp flashSpiIsErased function is not efficient, it only use byte by byte read and it is slow.
New strategy:

1. Read the first 4 bytes using byte by byte method
2. If the first 4 bytes shows it is erased, use DMA to ensure the whole block is erased

This can guarantee the efficiency of is erase detection for both used and erased blocks.

2.11 fix combines in #7472, no need to merge to 2.11 branch

Summary by CodeRabbit

• Performance Improvements
  • Improved flash erase verification speed by using bulk, DMA-friendly reads rather than checking every byte individually.
  • Added a two-phase sector verification approach to robustly confirm erased flash contents.
• Maintenance
  • Updated a bundled third-party dependency to a newer revision for ongoing compatibility and fixes.

[coderabbitai]

📝 Walkthrough

Walkthrough

flashSpiIsErased now uses staged DMA validation with a static sector buffer. The commented-out FTL garbage-collection call is removed, and the stb submodule pointer is updated.

Changes

Flash and FTL updates

Layer / File(s)	Summary
DMA-backed sector validation radio/src/targets/common/arm/stm32/spi_flash.cpp	Adds a DMA-capable sector buffer and replaces the full byte-by-byte scan with initial four-byte checks followed by DMA validation of the remaining bytes.
FTL read cleanup radio/src/drivers/frftl.cpp	Removes the commented-out doGC call from ftlRead; the existing bounds check and read path remain unchanged.
stb submodule update radio/src/thirdparty/stb	Updates the submodule pointer to a different upstream commit.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description covers the change, but it omits required template sections such as 'Summary of changes:' and an optional 'Fixes #' reference.	Add the template headings, especially 'Summary of changes:', and include a 'Fixes #' reference if applicable.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.
Title check	✅ Passed	The title accurately summarizes the main change: improving flashSpiIsErased erase-detection efficiency.

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[richardclli]

Tested, this change is already in my daily use radio.