Make the speech numbers reproducible instead of merely printed

A review of 5289cb4 found the ASR report was not honest arithmetic:

- "audio s" was the last segment's end, not the media length. The real
  container durations are 88.93 / 109.18 / 60.07 s, so the report understated
  the audio by 48 s and every speed cell was wrong.
- Decoding was unpinned, so a rerun was a new draw: WER moved 0.242 to 0.262
  and one video moved 0.278 to 0.583. Greedy single-temperature decoding with
  no carry-over now answers the same way twice, and the harness runs the pass
  twice to show it.
- The report claimed its WER was an upper bound. It is not a bound in either
  direction: a caption that drops words also hides words the model dropped.
  The denominators (201 / 296 / 36 reference words) are printed instead.
- "66 speech atoms" was bench-side arithmetic. The store now says 80 audio
  atoms carrying 563 word timestamps, read back after a real ingest.

Same silent-skip bug in a second place: a text file over max_text_file_bytes
returned an empty batch with no reason. It raises FileTooLarge with its size,
and a walk records that refusal like any other.

Also fixed from the same review: the Go search tool never published the audio
modality, cli_senses carried a second json_dump, the no-audio-track branch had
a test that fed it markdown, _seconds was an empty wrapper, and the docstring
quoted a speed no report measured.

Gate root cause: bench/tests imports the bench package by name with no root
conftest, so bare pytest could not collect it at all. pythonpath in pyproject
fixes both launch styles; 212 tests pass either way.

Verification: 212 passed (bare pytest and python -m pytest), ruff clean,
mypy clean on the six touched files with a cold cache, repo-wide mypy 24/6
unchanged from HEAD, pyright 0 errors on the touched files, go build/vet/test
and gofmt clean.

backend/src/ely_eye/cli_senses.py

@@ -9,12 +9,12 @@ from __future__ import annotations
 from collections.abc import Callable
 from dataclasses import asdict
 from pathlib import Path
-from typing import Any
 
 import typer
 
 from .asr import DEFERRED as SPEECH_DEFERRED
 from .asr import SpeechTranscriber
+from .cli_common import json_dump
 from .config import Settings
 from .db import Database
 from .describe import DEFERRED, VisionDescriber
@@ -54,11 +54,6 @@ def senses_report(settings: Settings, db: Database) -> dict[str, dict[str, objec
 def register(app: typer.Typer, settings: Callable[[], Settings]) -> None:
     """Add the media commands. The settings thunk lets one patch in a test redirect every command."""
 
-    def json_dump(value: Any) -> str:
-        import json
-
-        return json.dumps(value, ensure_ascii=False, indent=2)
-
     @app.command()
     def transcribe(path: Path) -> None:
         """Words with the instant each was spoken; the snapshot stays the operator's to supply."""