Rebrand to MifKit and add Converter registry

[fedyunin]

Summary

• Renames the project from MifMapXL → MifKit (1.0.4 → 1.1.0) so additional converters (KML/KMZ ↔ MapInfo, GeoJSON, Shapefile) fit naturally alongside the existing Excel exporter.
• Introduces a Converter contract and registry under src/core/converters/. New formats become one folder + one fixture + one entry — GUI and CLI will pick them up automatically.
• Wraps the existing Excel export engine as the first registered converter (mif-to-xlsx). Behavior is byte-identical to 1.0.4.

What changed

• Rename: package.json (name, version, appId, productName, artifactName), main window title, renderer header, README (with converter roadmap and redirect note), PROJECT_CONTEXT.md preamble, tmpdir prefixes in two tests.
• New src/core/converters/:
  • types.js — JSDoc Converter contract (id, inputs, outputs, options schema, async run)
  • registry.js — register/get/list/applyDefaults/validateOptions with strict shape validation
  • index.js — auto-registers built-in converters on first import
  • mif-to-xlsx/index.js — thin adapter over the existing convert.js engine
• UI and worker untouched in this PR. They keep calling convert.js directly — switching them to dispatch through the registry will land alongside the second converter so the contract is validated by two real implementations.

Tests

62 passing (55 existing + 7 new):

• registry rejects malformed converters, forbids duplicate ids
• applyDefaults merges schema defaults with explicit values
• validateOptions reports type/enum/range violations
• mif-to-xlsx is registered by default
• end-to-end dispatch through the registry produces a valid .xlsx for both folder and explicit-file inputs

npm test

Test plan

• npm test is green
• manual smoke: npm run dev, run a conversion, confirm same output as 1.0.4
• verify the auto-redirect from old GitHub URLs works for clones using MifMapXL.git

Follow-ups (next PRs)

1. feat/kml-to-mif — port the existing Python KML→MIF/MID converter to Node.js as the second registered converter; validates the contract with a real second implementation.
2. feat/dispatch-ui — switch renderer to render its options panel from the converter schema; switch worker to dispatch by converterId.
3. feat/cli — add bin/mifkit powered by the same registry (mifkit convert <id> <input> <output> --flags).

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