refactor: decompose discovery.py resolver monolith (1563 LOC)

[nikazzio]

Summary

src/universal_iiif_core/resolvers/discovery.py is 1563 LOC and growing. It contains search adapters for 8+ providers, manifest validation, probe logic, and result normalization — all in a single file.

#114 tracks the Studio UI monolith split but does not cover this core resolver file.

Current structure (single file)

Section	Approx LOC	Description
Archive.org search	~120	search_archive_org(), URL builder, result extraction
Harvard search	~80	search_harvard(), enrichment fetch
LOC search	~80	_search_loc(), pagination, result extraction
Europeana search	~60	_search_europeana()
Bodleian search	~50	_search_bodleian()
Institut de France	~40	search_institut()
Vatican search	~40	_search_vatican()
Gallica search	~80	_search_gallica(), smart search
Manifest validation	~100	validate_manifest_url(), _safe_probe()
Provider resolution	~100	resolve_provider_input(), smart_search()
Shared utilities	~80	Constants, headers, helper functions

Proposed split

src/universal_iiif_core/resolvers/
├── discovery.py          # Public API: smart_search(), resolve_provider_input(), validate_manifest_url()
├── search_adapters/      # One file per provider (archive_org.py, harvard.py, loc.py, etc.)
│   └── __init__.py       # Re-exports for backward compat
└── manifest_probe.py     # validate_manifest_url(), _safe_probe()

Relationship to other issues

• refactor: replace hardcoded discovery flows with a shared IIIF provider registry #104 (provider registry) introduces registry metadata — the search adapters here become registered implementations
• refactor: decompose Studio UI monoliths #114 (Studio UI split) is the parallel effort for UI files
• Discovery: Refactor UIX + Search Jobs (puntuale + aggregata) #108 (discovery UX) will add features to this code — splitting first reduces merge conflict risk

Acceptance criteria

• No file in resolvers/ exceeds ~300 LOC (excluding __init__.py re-exports)
• All existing tests pass unchanged
• Public API surface (smart_search, resolve_provider_input, validate_manifest_url) unchanged
• ruff check . --select C901 clean