docs(changelog): tighten unreleased notes

[Chemaclass]

🔗 Ticket

🤔 Background

The ## Unreleased entries added in #64 carried the reasoning behind each change — benchmark numbers, the mechanics of bashunit's doubles API, the full list of bumped action versions. That detail belongs in the PR and commit history. In a changelog it buries what a reader actually wants: what changed for them.

They were also hard-wrapped at 80 columns, which makes diffs noisy — rewording one sentence reflows the whole paragraph.

💡 Goal

Make the Unreleased section scannable, and stop hard-wrapping markdown prose.

🔖 Changes

• Cut the three ### Changed entries down to one line each, dropping benchmark figures and per-action version lists.
• Rewrote the ### Added entry to lead with the flags rather than the motivation.
• Dropped the empty ### Fixed heading, matching the convention 0.8.1 established.
• Unwrapped the prose. .editorconfig sets max_line_length for **.sh (120) only and never for **.md, so long lines pass make lint — confirmed locally.

Net: 30 lines to 9, with no information a changelog reader needs removed.

🖼️ Screenshots

BEFORE

### Changed

- `bashdep::_sha256` resolves `shasum`/`sha256sum` once per run (cached
  in `BASHDEP_SHA_TOOL`) instead of re-probing on every call, mirroring
  how `BASHDEP_DOWNLOADER` already pins `curl`/`wget` for a whole run, so
  every checksum in a run is computed by the same tool. It also slices
  the digest with parameter expansion, which keeps `awk` out of that
  path. Measured as performance-neutral (150 digests: 1.51s before and
  after) — this is a consistency change, not a speedup.

AFTER

### Changed

- `bashdep::_sha256` resolves `shasum`/`sha256sum` once per run rather than probing on every call, so all checksums in a run use the same tool — matching how `BASHDEP_DOWNLOADER` already pins `curl`/`wget`.

✅ Verification

make lint clean. Documentation only — no code touched, so the tests and ShellCheck jobs skip via the existing paths filter.