v0.3.1: Clearer investigation reports

v0.3.1 Release Notes

v0.3.1 makes investigation reports easier to scan and brings the release binary
in line with the README examples. The report starts with the main finding,
then shows contributor tables, coverage, and concise notes.

Try It

With the verified binary installed, run this from a repository checkout:

fleetdiff investigate --before examples/sessions/data/before \
  --after examples/sessions/data/after --expected app

The synthetic sessions example opens with:

1 of 8 tracked sessions flagged for review: 90.91% of attributed tokens.
Reported tokens: 400 -> 3300; model attempts: 4 -> 13.
  +1592 tokens from attempt count; +1308 tokens from tokens per attempt.

Token-change contributions display as whole tokens. JSON retains the calculated
precision; token-per-attempt averages and share bounds keep their existing display
precision. Unequal share bounds remain outward-rounded ranges.

Upgrade

Replace the binary after following the verification instructions.
The four supported targets remain Linux and macOS on AMD64 and ARM64.

Existing summary files, flags, accounting, and JSON version 1 are unchanged.
Session flags still require a lower-bound share strictly above the configured
threshold and complete relevant observations. No collector upgrade or window
reset is needed. Text consumers should use --format json for structured data;
the human-readable report layout has changed.