agents: preserve indexed Pydantic text replacements

[yisding]

Problem

PR #961 restored initial PydanticAI TextPart content, but the bridge still
normalized every part start as an append-only text_delta and discarded
PartStartEvent.index. PydanticAI may emit another start for the same index as
a replacement, so consumers could retain stale text or append duplicate text.

What changed

• extend AgentBridgeEvent with indexed text events and a text_replace kind
• preserve PydanticAI part indexes for starts and deltas, including repeated
  same-index starts and graph-local index namespaces
• use one canonical text-part accumulator across bridge, runner, stage, text,
  and voice consumers so transcripts and history preserve replacement semantics
• expose replacement metadata through AgentDelta and journal records
• define TTS behavior: repair replacements while text is buffered; once speech
  is admitted, cancel and clear stale playback and suppress further TTS for the
  turn while allowing the corrected transcript and history to complete
• document the bridge contract and add regression coverage for same-index starts,
  buffering, playback cutoff, graph streams, and consumer history

Impact

Ordinary unindexed text_delta streams remain append-only. Indexed bridges must
use indexed deltas consistently within a turn. A divergent replacement after
TTS admission fails closed instead of replaying corrected text over speech the
caller may already have heard.

Verification

• 368 passed across the focused bridge, PydanticAI, stage, session consumer,
  event, and lifecycle suites
• just guard-contracts (218 passed)
• .venv/bin/ruff check src tests
• .venv/bin/ruff format --check src tests
• .venv/bin/mypy src/easycat (289 source files)

Follow-up to #961.

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