[daplein]

Adds service-level aliases, modelled on synonym maps (#79).

• GET/POST /aliases, GET/PUT/DELETE aliases('x') and aliases/x
• File storage (<name>.alias.json), same path-character guard as indexes and synonym maps
• An alias targets exactly one existing index and cannot share a name with an index (and vice versa, 409)
• Document routes (search, get, count, suggest, autocomplete, index) resolve aliases via IIndexResolver; /indexes routes do not, as in Azure
• A dangling alias answers 404 like a missing index
• aliasesCount in /servicestats reports real usage
• Spec in docs/specs/aliases.md, README section

Tests: AliasTests (unit) and AliasIntegrationTests (CRUD, validation, resolution, re-pointing, dangling alias, stats).

[daplein]

This is the largest of four independent PRs (#94, #96 and #97 are the others, each branching off main), so merge order doesn't matter. If it helps review, #94 and #97 are small and can go first. Happy to answer any questions or split this up if that would make it easier to review.