fix(ch32v): honor board ISA and ABI settings

[zackees]

Closes #1106

Summary:

• load board build.march and build.mabi into BoardConfig
• normalize and apply CH32V ISA/ABI flags to compile and link commands
• add focused normalization, propagation, and compiler tests

Validation:

• soldr rustfmt --check on all changed Rust files
• soldr cargo check -p fbuild-build-mcu -p fbuild-config --lib (shared Windows toolchain did not return within the local validation window)

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📥 Commits

Reviewing files that changed from the base of the PR and between c46bd3d and c54edd3.

📒 Files selected for processing (6)

• crates/fbuild-build-mcu/src/ch32v/ch32v_compiler.rs
• crates/fbuild-build-mcu/src/ch32v/mcu_config.rs
• crates/fbuild-build-mcu/src/ch32v/orchestrator.rs
• crates/fbuild-config/src/board/db.rs
• crates/fbuild-config/src/board/loaders.rs
• crates/fbuild-config/src/board/types.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/ch32v-board-isa

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