refactor: delete unwired ASRSegmentModel + StreamingTextReconciler (QUA-263/QUA-265)

[giraphant]

Summary

Deletes the two "reserved, not-yet-wired" design assets kept under ARCHITECTURE.md §7. Both sat unwired since landing, with zero production evidence for their differentiators (nonstream_result=true: 0 of 2935 streamed results across two log files; pauseBoundary: no consumer). Net -961 lines.

• Delete ASRSegmentModel.swift (248), StreamingTextReconciler.swift (114) and their test suites (431).
• Inline the shrink-rescue heuristic — the one live piece, formerly ASRHypothesis.looksLikeNewUtterance shared with the shipping rescue path — directly into DoubaoResultState.ingest. Behavior unchanged.
• Trim ASRPipelineIntegrationTests to the two live final-path tests (formatter → corrector, as handleFinal composes them); the five cold-layer tests leave with the code.
• Rewrite ARCHITECTURE.md §7 as a removal record: resurrection triggers (nonstream late revisions observed / third engine / pause-aware boundaries / incremental injection) and the domain decisions worth not re-deriving (empty-snapshot hold, grapheme ≠ backspace count, scalar-exact prefix matching, no sentence-max forced boundary, per-engine event mapping).

Resurrection path: git history, or rewrite against the §7 notes.

Test plan

• swift test: 322 tests, 0 failures.
• Pure logic + docs only — recording pipeline untouched, no make install / real-device pass needed.

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