[nihalpasham]

Follow-up to #233. That PR landed the basic line-table/full-debug path; this fixes the remaining cross-file span case properly and documents the new debug-info modes in the book.

What changes

Before, an instruction inside one function only got a debug location if its source span came from the same file as the function scope. That avoided wrong metadata, but dropped useful spans from helpers/macros/inlined code.

This PR keeps those spans by creating a file-specific debug scope first:

kernel.rs function scope
  |
  +-- thread.rs file scope
        |
        +-- line 292, column 19

In LLVM metadata terms, that is:

!thread_scope = !DILexicalBlockFile(scope: !kernel_scope, file: !thread_file, discriminator: 0)
!loc = !DILocation(line: 292, column: 19, scope: !thread_scope)

For explicit Pliron callsite locations, the exporter also emits the call chain:

!call = !DILocation(line: 39, column: 13, scope: !kernel_scope)
!loc = !DILocation(line: 292, column: 19, scope: !thread_scope, inlinedAt: !call)

So cuda-gdb can see: we are at thread.rs:292, reached from kernel.rs:39.

Book docs

Adds user-facing docs for:

• CUDA_OXIDE_DEBUG=line-tables
• CUDA_OXIDE_DEBUG=full
• what source debugging supports today
• what basic local/argument inspection supports today
• the simple compiler flow: rustc Span -> Pliron Location -> LLVM !dbg -> PTX/DWARF

Also updates the supported-features matrix so source debugging is marked full, while local/argument inspection is marked partial.

Tests

• cargo fmt --all -- --check
• cargo test -p llvm-export
• cargo check --workspace
• attempted make -C cuda-oxide-book html, but this local environment does not have sphinx-build installed

Tracker: #226