[kvdb] Support 256bit write granularity for kvdb

[Baseline-K]

• Support 256bit write granularity
  The new stm32h7xx chips can only write double quad words, i.e. 256bits.

*Update fdb_tsdb.c to include 256bit option
*Update fdb_cfg.h to include 256it option.

[Copilot]

Pull request overview

Adds 256-bit flash write granularity support for KVDB to accommodate STM32H7 devices that require 256-bit (double quadword) programming, and updates related configuration/documentation and TSDB compile-time guards.

Changes:

• Allow FDB_WRITE_GRAN == 256 in KVDB configuration checks.
• Add 256-bit-specific alignment padding in KVDB sector/KV header structs.
• Update TSDB compile-time error guard and the config template comment to mention 256-bit granularity.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/fdb_tsdb.c	Extends TSDB’s compile-time unsupported-granularity check to include 256-bit.
src/fdb_kvdb.c	Allows 256-bit granularity and adjusts header struct padding for alignment.
inc/fdb_cfg_template.h	Documents 256-bit write granularity option for STM32H7 in the template config.

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[Copilot]

The TSDB compile-time error string could be made clearer/grammatically correct (e.g. "Flash write granularities 64/128/256 bits are not supported in TSDB yet"). This helps users quickly understand which configurations are invalid.

Suggested change

	#error "Flash 64 or 128 or 256 bits write granularity is not supported in TSDB yet!"
	#error "Flash write granularities 64/128/256 bits are not supported in TSDB yet."