Refactor: Consistent branch naming with ledger:branch format

[bplatz]

Standardize branch naming to ledger:branch; reserve @ for time travel

Summary

Standardizes branch naming to ledger:branch, removes branch segments from content‑addressed paths, and scopes nameservice records by branch. Consolidates branch handling and reserves @ for time travel (PR #1088).

Motivation

• Support upcoming branching features with a consistent, parseable ledger:branch format.
• Keep ledger names URL‑safe so ledgers can be used as full IRIs when hosted.
• Reserve @ exclusively for time travel to align with forthcoming query semantics.
• Streamline branch handling across db, ledger objects for more consistency

Key Changes

• Branch format
  • ledger-name:branch-name (e.g., my-ledger:main)
  • : reserved from ledger names; @ reserved for time travel (e.g., my-ledger:main@t:42)
  • Branch names disallow / to avoid unintended subdirectories
• Paths
  • Content‑addressed artifacts (commit/txn/index) no longer include branch
    • Example: /my-ledger/commit/[hash].json (deduped across branches)
  • Nameservice files are per‑branch subfiles
    • Example: /ns@v1/my-ledger/main.json (supports ledger names with /)
• API and internals
  • fluree.db.api/create rejects : in ledger names (:db/invalid-ledger-name)
  • New fluree.db.util.ledger with ledger-base-name, ledger-branch, ledger-parts
  • Removed redundant branch tracking from Ledger; fixed circular deps
  • Commit storage now uses ledger-name (not alias)

Impact

• Ledger names cannot contain :
• Branch names cannot contain /
• External tools relying on branch-in-path or old nameservice locations must update
• No data re-encoding; hashes remain stable and deduplicated

Testing

• New/updated tests for name validation, nameservice layout, and path conventions
• All tests passing; no lint warnings

Related Work

• Builds on time travel proposal in PR #1088, which adds @-based query time travel:
  • Supports @t:<tx>, @iso:<timestamp>, @sha:<commit>, works with ledger:branch
  • @ takes precedence over legacy ?t=...; supports git‑style SHA prefixes

[zonotope]

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