Add sniff_function support to ImportPlugin for version-aware dispatch

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Summary

• Adds an optional sniff_function attribute to PluginData and ImportPlugin so multiple importers can share a file extension and be selected by file content.
• Updates both GUI (dbloader.py) and CLI (arghandler.py) dispatch to prefer a sniff-matched plugin over the first-match fallback.
• Adds 10 unit tests covering all dispatch paths.

Motivation

The .ged extension is currently mapped to a single GEDCOM importer.
The GEDCOM 7 addon (PR #744) needs to register a second importer for .ged files and have it chosen automatically when the file header identifies the file as GEDCOM 7. GEDCOM 7 requires a version string in the header, so detection is trivial — the only missing piece was a hook in core.

How it works

A plugin declares a sniff_function by name in its .gpr.py:

plg.sniff_function = "sniffData"

The function accepts a filename and returns True when the plugin should handle it:

def sniffData(filename):
    with open(filename, "rb") as f:
        for line in itertools.islice(f, 20):
            if b"VERS 7." in line:
                return True
    return False

Plugins without a sniff_function continue to work exactly as before.

Test plan

• GRAMPS_RESOURCES=. python3 -m unittest discover -p "*_test.py" gramps/gen/plug/test — all 10 tests pass
• black run on all changed files — no reformatting needed
• Manual smoke test: import a GEDCOM 5.5 file and verify the built-in importer is still selected
• Manual smoke test: install GEDCOM 7 addon and verify a GEDCOM 7 file routes to it

🤖 Generated with Claude Code

[kulath]

It would be really nice if the Import code were converted to support GEDCOM extension addons as #0010550: Refactor Gramps GEDCOM import so as to support GEDCOM extension addons at the same time as this change, so that it is simpler to write alternative importers.

[emyoulation]

I've had updated vCard importer and export plugins wish-coded that add support beyond the original v3.0 format for 5.2 and 6.0 Gramps. (It seems most smart phones still use version 2.1 .vcf files.) Do you want to use them to exercise the sniffer?

I'd like to roll them into 6.1 but testing as addons might tell us something about integrating. The AI tried building automatic hiding of the superseded into the .gpr.py file. But that did not work.

https://github.com/emyoulation/CuratedGrampsPlugins/tree/main/gramps60/download

• .vcf format: Enhanced version Supports vCard versions 1, 2.1, 3.0, and 4.0. Unhandled properties are preserved as Person attributes. ( .vcf Variant Call Format (DNA) files are detected and rejected.)
• new .webp thumbnailer
• future .avif thumbnailer
• future .vcf that supports more fields. (Such as embedded avatar bitmaps.)
• future .ged 5.5.1 that does not mangle unhandled GEDCOM tags as formatted Notes. Preserve them as raw Attributes for easier post-processing and dialect export.