fix: replace widget file FFI with simplifile

[GG-O-BP]

Closes #14.

Summary

Moves the widget-definition file boundary (src/glendix/define/file_boundary.gleam) off the handwritten Node fs / JSON.parse adapter and onto simplifile (existence checks, UTF-8 read/write) and gleam_json (a small widgetName decoder), mapping simplifile.FileError / gleam_json.DecodeError into the existing public FileError taxonomy in pure Gleam.

Acceptance criteria

• read / write / find_widget_xml delegate to simplifile and gleam_json; no handwritten fs / JSON.parse remains in this boundary.
• src/glendix/define/file_boundary_ffi.mjs removed (no longer referenced).
• Public FileError variants and their path / reason fields are unchanged.
• Tests cover missing package.json, missing/empty widgetName, missing XML file, successful read/write, and read/write failure paths.
• FFI contract checks report no stale external declarations or exports.
• glendix -> mendraw retains its declared Hex dependency source form.

Behavior preservation & edge cases

• Missing / empty / whitespace-only widgetName -> WidgetNameWasNotDeclared.
• Missing src/<widgetName>.xml -> WidgetXmlWasNotFound.
• Read / write OS failures -> FileCouldNotBeRead / FileCouldNotBeWritten.
• reason text is now package-derived and deterministic (via simplifile.describe_error and a stable gleam_json.DecodeError description) instead of raw Node error.message.
• Malformed JSON -> FileCouldNotBeRead with a deterministic reason.
• Non-object valid JSON and null / non-string widgetName -> WidgetNameWasNotDeclared, matching the original adapter's observable behavior while also removing a latent uncaught TypeError the old adapter threw for a top-level null document.
• A directory at the XML path is not accepted as the widget file.

Verification

Validated after rebasing onto main at 5071385:

• gleam format --check src test, gleam check, gleam build --warnings-as-errors, gleam docs build
• gleam test --runtime bun — 51 tests passed
• ./scripts/check-all.sh — style, boundaries, 313 FFI export contracts, and generated-source diff passed
• ./scripts/family.sh widget-build — valid MPK produced
• ./scripts/family.sh lustre-e2e — browser smoke check passed
• Mendix prepare/build plus the managed runtime/browser E2E — passed using explicit alternate ports because host port 8080 is occupied by an unrelated root-owned listener

./scripts/test-all.sh was also attempted after the rebase. Dependency-mode and Codex tests plus the glendam, glendix, and mendraw standalone consumers passed; the final fresh mxpak consumer resolution was blocked by the shared Hex API rate limit. Direct package/browser suites and the end-to-end routes passed independently.