fix(base-db): handle included SystemVerilog files as include-only units

[hongjr03]

Closes #27, should be merge after #32.

When a .sv file is reached through a SystemVerilog `include and is also discovered by the workspace scan as a standalone source file, semantic compilation can add the same physical file twice.

In projects without an explicit config, Vizsla currently treats scanned .sv files as semantic roots. For a layout like:

// a_pkg.sv
`include "z_frag.sv"

z_frag.sv is first assigned by slang while preprocessing a_pkg.sv, then Vizsla later tries to add z_frag.sv again as an independent compilation root. slang rejects the duplicate source-manager path and aborts with:

internal compiler error: 'Buffer with the given path has already been assigned to the source manager'

This crashes the language server process.

Change

This PR builds a preprocessor file index from slang directive trivia and uses it to classify literal include targets.

• Adds a preprocessor index for macro directives, including literal `include targets.
• Builds compile units per source root / compilation profile.
• Treats .sv files reached by literal includes as include-only units.
• Provides include-only files to slang through include buffers.
• Excludes include-only .sv files from standalone semantic roots.
• Keeps diagnostics mapped back to included files.

Follow-up

While investigating the LSP crashes, we found a second, separate crash class: semantic requests can also abort when stale borrowed slang syntax nodes outlive their backing SyntaxTree.

That issue is not caused by duplicate include buffers and is intentionally not fixed here. It is handled in #31, which makes semantics syntax ownership explicit and replaces long-lived borrowed syntax nodes/tokens with stable syntax pointers.