fix(transpiler): include diagnostic locations

[Photon101]

PR summary

Preserve line/column information from type-check diagnostics when transpileCellFile() returns build errors.

Change type

• Bug fix
• Feature
• Refactoring
• Docs
• Test/build/CI
• Other

Background / problem

transpileCellFile() already receives diagnostics with positions, but mapped them down to message strings. That meant cell build --json could not show where type-check failures came from.

Changes

• Format compiler diagnostics as line N, column M: message before returning them in TranspileResult.errors.
• Keep the existing errors: string[] shape so current CLI and runtime consumers continue to work.
• Update fixtures/broken.cell to exercise a parse-valid, type-check-invalid input and assert the location in transpiler.test.ts.

Impact area

• Runtime
• Python Bridge
• Viewer
• DSL / Compiler
• Docs

Verification

npm run typecheck
npm test
npm run pack:smoke
node --import tsx --test transpiler.test.ts
npm run cell:build -- --json fixtures/broken.cell
npm run cell:build -- --json ../examples/validator.cell
npm run cell:build -- fixtures/broken.cell
git diff --check

The broken build command exits non-zero as expected and now reports line 1, column 1 in both JSON and human output.

Test checklist

• Local tests were run
• Tests were added/updated for the changed behavior
• Docs were updated where applicable
• Migration guide was added for breaking changes

Screenshots / logs

Not applicable.

Related issue

Closes #32