feat: transcriber DX improvements and RealTimeTranscriber rename

[ccampbell-aai]

Stacked on #231 (base is ccampbell/api-key-constructors; retargets to master once that merges). Additive DX improvements across the transcribers plus the streaming rename. No existing behavior changes; all new parameters default to current behavior.

• README: "Choosing a transcriber" decision table for the four transcriber classes.
• Config guards: passing a SyncTranscriptionConfig to Transcriber (or the reverse) raises a TypeError naming the right class at the constructor, config setter, and every transcribe call — instead of a bare AttributeError deep in request building. TranscriptionConfig gains a class docstring.
• Input handling: Transcriber now accepts pathlib.Path and bytearray like its three siblings; unsupported input raises unsupported audio input type: ... instead of a raw httpx error; type aliases match runtime.
• poll_timeout= (keyword-only, seconds, default None = today's unbounded polling) on Transcriber.transcribe/transcribe_async and AsyncTranscriber.transcribe; on expiry raises TranscriptError naming the transcript id and last status so it can be resumed via get_by_id.
• Failure-contract docstrings: the polling transcribe methods now document that a failed transcription returns a status=error transcript rather than raising.
• Sync API errors: _error_from_response falls back to a plain {"error": ...} body instead of degrading to "failed with status NNN".
• aai.streaming is reachable as an attribute via lazy import (no websockets cost at import assemblyai time).
• Rename: StreamingClient → RealTimeTranscriber, carried through the public streaming surface (AsyncRealTimeTranscriber, RealTimeTranscriberOptions, RealTimeParameters, RealTimeSessionParameters, RealTimeEvents, RealTimeError, RealTimeErrorCodes). Every former Streaming* name remains a plain alias to the same object — isinstance and identity hold in both directions, imports stay warning-free, and the 157 existing streaming tests run unmodified against the old names as compat coverage. README and CLAUDE.md updated to the new names with an alias note.

Bumps the version to 1.0.0 — this PR is the major-release bump. (Written as exactly 1.0.0: PEP 440 normalizes away the trailing-.00 house style, so a v1.0.0 release tag can only match this form.)

Tests: full suite 463 passed (405 pre-existing untouched + 58 in tests/unit/test_dx.py).

Note for reviewers: the SDK historically shipped RealtimeTranscriber (lowercase t) as the old v2 realtime client — the new name differs only in the capital T. Flagging for a deliberate call on whether that collision is acceptable before this lands.

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