Add sniff_function support to ImportPlugin for version-aware dispatch

[dsblank]

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 (except minor spacing in _import.py)
• 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

[Nick-Hall]

@dsblank Please don't create branches in origin. Use you own fork when creating pull requests.

[dsblank]

Please don't create branches in origin. Use you own fork when creating pull requests.

I'll add that to AGENTS.md