PlantUML linker fails on generated .fbs.bin with UTF-8 error although source .puml is ASCII-only

## Summary

A sequence diagram parses successfully with the PlantUML parser step, but the later linker step fails while reading the generated `.fbs.bin` with a UTF-8 error in the `source_file` field.

## Observed behavior

Warning / failure shape:

`Failed to parse FlatBuffer ... Utf8 error ... while verifying table field 'source_file'`

Affected source:
`attribute_access_getter_sequence.puml`

Generated artifact:
`attribute_access_getter_sequence.fbs.bin`

## Expected behavior

If the source `.puml` parses successfully, the generated FlatBuffer should also be consumable by the linker.

## Why this looks like a tooling bug

- The `.puml` source is ASCII-only.
- `puml_cli` parses it successfully.
- The failure appears only in the linker while verifying the generated FlatBuffer.

## Impact

Valid diagrams can still trigger downstream linker warnings/failures, making architectural-design validation noisy and less trustworthy.

## Narrow repro

Build the architectural / dependable-element docs containing the affected sequence diagram.

## Notes

This may be adjacent to the ongoing PlantUML idmap/linker refactor work (#290, #321, #322, #323), but those PRs do not appear to describe this UTF-8 / FlatBuffer failure explicitly.

## Suspected area

Mismatch between:
- PlantUML parser output
- FlatBuffer schema/content
- linker-side verification expectations