Add Gigapan image support

[lovasoa]

Summary

• add pure Gigapan discovery for viewer pages and .kml metadata
• build the full-resolution 256px tile grid using Gigapan's /get_ge_tile endpoint
• register Gigapan across the core, CLI, protocol artifacts, and documentation

Closes lovasoa/dezoomify-rs#356

Verification

• cargo test -p dezoomify-core
• cargo xtask test core --parity
• cargo test -p dezoomify-protocol
• cargo xtask test protocol
• cargo test -p dezoomify-cli
• cargo clippy --workspace --all-targets -- -D warnings
• cargo xtask protocol generate --check
• cargo xtask fixtures verify

The repository-wide check is otherwise clean; cargo xtask check is blocked only because cargo-deny 0.20.2 is not installed in the environment. The full workspace test also reaches unrelated JavaScript suites that require missing esbuild and vite dependencies.

[lovasoa]

@CharlyAlonso can you test this on https://pr-1009.dezoomify.pages.dev/beta/ ? Gigapan is too slow to answer for me

[CharlyAlonso]

Hello,

I have tested it using various browsers and different source IP addresses, and in every case, I get the attached error.

Bye,

[CharlyAlonso]

I would add:
It is also true that, right now, the response time from https://gigapan.com/ is extremely slow, and that might be a factor.

[lovasoa]

I am switching this to draft. If noone can actually reach this site, then there is no point in supporting it. If someone can reach it, they can try this pr and report their results to unblock this