refactor: drop the bundler backend contract

[dangreen]

Drops the bundler backend contract entirely - image variants are always generated with the sharp generator, proxy adapters build urls at runtime instead (see #13).

• generateSrcSetModule creates SrcSetGenerator directly; srcsetBackend, SrcSetBackend, SrcSetBackendFactory and the backend option are removed
• SrcSetBackendImage folded into SrcSetModuleEntry with url narrowed to SrcSetImagePaths: plain string urls existed only for url backends, so the dead branch of toUrlExpression is dropped too
• EmitImage and SrcSetImagePaths stay - the emit machinery of the integrations themselves
• Backend mentions cleaned from the READMEs and metadata: the root feature list and the imgproxy descriptions are runtime-only now
• Reviewed locally with Codex: docs tails and dead SrcSetModuleEntry re-exports from the integrations found and removed

🤖 Generated with Claude Code

[coveralls]

Coverage Report for CI Build 31256271347

Coverage decreased (-0.6%) to 90.798%

Details

• Coverage decreased (-0.6%) from the base build.
• Patch coverage: 3 of 3 lines across 1 file are fully covered (100%).
• 4 coverage regressions across 2 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

4 previously-covered lines in 2 files lost coverage.

File	Lines Losing Coverage	Coverage
packages/loader/src/paths.ts	2	30.0%
packages/loader/src/template.ts	2	90.48%

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Coverage Stats

Relevant Lines:	656
Covered Lines:	610
Line Coverage:	92.99%
Relevant Branches:	485
Covered Branches:	426
Branch Coverage:	87.84%
Branches in Coverage %:	Yes
Coverage Strength:	79553.21 hits per line

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💛 - Coveralls