Fix board source path overrides

[NetVar1337]

Motivation

• Some FPGA board repositories place Vivado TCL scripts at the project root while keeping src/ip under a nested directory, which prevented correct source/IP lookup for those boards.
• The ZDMA board in the upstream pcileech-fpga layout uses a nested 100T source tree that needs to be referenced separately from the TCL scripts.

Description

• Add an optional source_sub_dir board metadata field (SourceSubDir in Board) and a SourceBasePath helper so source/IP paths can be resolved independently of TCL/project paths.
• Change SrcPath and IPPath to use SourceBasePath and keep TCLPath/BuildTCLPath behavior unchanged for TCL files.
• Update internal/board/boards.json to set source_sub_dir for ZDMA so its sources live under ZDMA/100T.
• Add and update tests (TestBoard_SourceSubDirPaths and adjustments to existing board path tests) in internal/board to cover the split TCL vs source layout.

Testing

• Ran go test ./... and all tests completed successfully.
• Added board path tests exercising SourceBasePath, SrcPath, IPPath, and TCLPath which passed under the test run.

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