Bump GOBL to v0.508.1 and add conversions via the convert register - #63

Upgrade GOBL to v0.508.0 and offer conversions between GOBL and external
formats through its new convert package. Only formats registered with the
register are offered, replacing the hand-maintained format list of #22.

Modules:

  gobl             v0.507.0 -> v0.508.0
  gobl.ar.arca     new      v0.0.3   (ar-arca-v4 moved out of core)
  gobl.co.dian     new      v0.0.3   (co-dian-v2 moved out of core)
  gobl.ubl         new      v0.86.0  (registers ubl+ formats)
  gobl.cii         new      v0.69.0  (registers cii+ formats)
  gobl.fi.finvoice v0.0.2  -> v0.0.6
  gobl.it.sdi      v0.81.0 -> v0.83.0
  gobl.mx.cfdi     v0.64.0 -> v0.68.0
  gobl.pl.ksef     v0.45.0 -> v0.47.0

GOBL 0.508.0 changes handled:

- The ARCA and DIAN addons now live in their own modules; blank-import
  both in the bundle so the approved addon test passes.
- data/addons was removed. Addon definitions are already served from the
  registry, so only the TestListDataDir subtest reading it is dropped.
- Shortened schema descriptions and the new tax/filter schema: refresh
  the correction options snapshots and the bulk schema expectations.

Conversions:

- internal/ops: Formats, Import and Export wrap the convert register.
  Export envelops documents and calculates unsigned envelopes first, so
  drafts can be exported directly.
- CLI: `gobl convert` exports with --to (keys in order of preference),
  imports with format detection otherwise (--from limits it), and lists
  the formats with --list.
- API: GET /v0/formats, POST /v0/import and POST /v0/export?format=,
  taking raw bodies. The format used is returned in a GOBL-Format header,
  exposed through CORS. Unescaped "+" in keys is accepted.
- Editor: an Export picker lists the registered export formats and shows
  the result in a read-only XML pane beside the editor, via /v0/export.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Rework the editor around three modes:

- Build: as before, the GOBL source is replaced with the built document.
- Export: the source is left untouched; /v0/export builds it internally
  and the result is shown in a read-only viewer. The formats offered are
  those registered with the convert package whose export schemas include
  the document's schema and whose addons are all declared by it, after
  expanding addon requirements as GOBL does when building.
- Import: a separate XML input is converted through /v0/import, and the
  resulting envelope is shown in the viewer.

Add bare-minimum example documents for the US, UK, Spain (with and without
VERI*FACTU), Germany (EN 16931 and XRechnung), France and the Netherlands
(EN 16931), Italy (SDI), Portugal (SAF-T) and Mexico (CFDI). Their region
and label are derived from the regime and addon registries, loaded lazily
so externally implemented addons are registered first. In import mode an
example is offered as the output of its first export format.

Tests build every example and export it into every format the editor
would offer it. ubl+peppol+bis-v3 is recorded as a known gap: it also
exports bill/status, so the EN 16931 addon it needs for invoices is only
checked in Accepts and is missing from the register's metadata.

The mode, example and format are remembered in localStorage.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Upgrade to GOBL v0.508.1 and add the converters that register regional
formats with its convert package:

  gobl             v0.508.0 -> v0.508.1
  gobl.ubl         v0.86.0  -> v0.87.0
  gobl.cii         v0.69.0  -> v0.70.0
  gobl.de.xinvoice new      v0.0.6  (XRechnung, ZUGFeRD)
  gobl.fr.ctc      v0.0.8   -> v0.0.9  (CTC, Factur-X, Chorus Pro, CDAR)
  gobl.sa.zatca    v0.0.4   -> v0.0.5  (ZATCA)
  gobl.it.sdi      v0.83.0  -> v0.84.0
  gobl.pt.saft     v0.0.8   -> v0.0.9

gobl.pt.saft v0.0.9 requires Go 1.26.1, so the go directive moves up and
the Docker builder image moves to golang:1.26-alpine (its GOTOOLCHAIN is
local, so the build would otherwise fail).

GOBL v0.508.1 removes Format.Addons, and converters now explain in
Accepts why they refuse a document, as rules faults. Only a converter can
tell whether it accepts a document, and the register only asks while
exporting, so:

- ops.CheckFormats reports, for each format that exports the document's
  schema, whether it accepts it, by exporting a copy of the envelope, and
  the refusal reasons. It takes about 9ms for all 21 export formats.
- POST /v0/formats exposes the checks through the API and OpenAPI spec.
- The editor's export picker lists the formats the server reports as
  accepting the source, rechecking and re-exporting as it is edited,
  replacing the client-side addon matching. The ubl+peppol+bis-v3 gap
  this had goes away.
- Examples carry the formats that accept them, those specific to their
  country first, so loading one selects its most specific format and
  import samples use it.

Add bare-minimum examples for ZUGFeRD, Factur-X, French CTC, Chorus Pro
and ZATCA, so that every invoice export format is accepted by at least
one example, which a test now checks.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Replace the build, export and import modes with two:

- Build: the GOBL editor fills the page, with the example picker, envelop
  option and Build button directly above it.
- Convert: the GOBL editor sits beside an editable document in another
  format. The GOBL pane's bar holds the example picker, the formats that
  accept the document and Export, which writes it into the other editor.
  The other pane's bar holds Import, which replaces the GOBL document, and
  samples made by exporting the examples into their most specific format.
  ⌘/Ctrl+Enter runs the action of the focused editor.

The top bar keeps only the title, the mode switch and the dark mode
toggle, and the read-only viewer goes, as results are written into the
editors. Formats are still checked as the GOBL document is edited, but
it is no longer re-exported, so a document pasted on the other side is
not overwritten. Stored export and import modes map onto convert.

/v0/import now returns the detected format in the GOBL-Format header,
via ops.Import, so the other pane can be labelled and the format picker
defaults to exporting back into the format a document was imported from.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>