Fold OptimizedTagMap into a final class TagMap (drop the interface)

TagMap was an interface with a single implementation, OptimizedTagMap. The
split was vestigial scaffolding from when a second (HashMap-backed) impl
existed; with one impl it is false generalization. Collapse them into one
`public final class TagMap`:

- The interface's abstract method declarations are removed; OptimizedTagMap's
  bodies become TagMap's methods.
- Nested types that were implicitly `public static` in the interface
  (EntryChange, EntryRemoval, EntryReader, Entry, Ledger) are now written out
  explicitly as `public static`.
- Static factories (create/fromMap/ledger/...) and the EMPTY constant become
  explicit `public static` members; the EmptyHolder lazy-init note is updated
  now that there is no interface<->impl class-init cycle.
- putAll(TagMap) loses its `instanceof` dispatch (always true once there is one
  class) and calls the fast path directly.

No behavior change; motivation is code simplicity, not performance (a single
final class is monomorphic by construction, but CHA already devirtualized the
sole impl). Public API is preserved, so callers are unchanged; the 3 tests that
referenced OptimizedTagMap now reference TagMap.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dougqh